MCAT Biological and Biochemical Foundations of Living Systems Quiz: 1c Mutations Inborn Errors
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1c Mutations Inborn ErrorsQuestion 1 of 20

A patient has episodic weakness triggered by fasting. Labs during an episode show hypoketotic hypoglycemia and elevated medium-chain acylcarnitines. Sequencing reveals a mutation that reduces medium-chain acyl-CoA dehydrogenase (MCAD) activity. Using the concept of impaired ββ-oxidation limiting acetyl-CoA production, which outcome is most consistent?

Increased oxidation of very-long-chain fatty acids in peroxisomes as the sole cause of symptoms
Excess ketone body production during fasting due to increased fatty acid oxidation
Primary hyperammonemia due to direct inhibition of the urea cycle by MCAD substrates
Reduced ability to generate acetyl-CoA for ketogenesis during fasting, leading to low ketone bodies despite hypoglycemia
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MCAT Biological and Biochemical Foundations of Living Systems Quiz

MCAT Biological and Biochemical Foundations of Living Systems Quiz: 1c Mutations Inborn Errors

Practice 1c Mutations Inborn Errors in MCAT Biological and Biochemical Foundations of Living Systems with focused quiz questions that help you check what you know, review explanations, and build confidence with test-style prompts.

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This quiz focuses on 1c Mutations Inborn Errors, giving you a quick way to practice the rules, question types, and explanations that matter most for MCAT Biological and Biochemical Foundations of Living Systems.

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Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

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Question 1

A patient has episodic weakness triggered by fasting. Labs during an episode show hypoketotic hypoglycemia and elevated medium-chain acylcarnitines. Sequencing reveals a mutation that reduces medium-chain acyl-CoA dehydrogenase (MCAD) activity. Using the concept of impaired ββ-oxidation limiting acetyl-CoA production, which outcome is most consistent?

  1. Increased oxidation of very-long-chain fatty acids in peroxisomes as the sole cause of symptoms
  2. Excess ketone body production during fasting due to increased fatty acid oxidation
  3. Primary hyperammonemia due to direct inhibition of the urea cycle by MCAD substrates
  4. Reduced ability to generate acetyl-CoA for ketogenesis during fasting, leading to low ketone bodies despite hypoglycemia (correct answer)
Explanation: This question assesses fatty acid oxidation disorders from dehydrogenase mutations. The principle is that defects in beta-oxidation limit acetyl-CoA production, impairing ketogenesis and causing hypoketotic hypoglycemia during fasting. The MCAD mutation reduces medium-chain fatty acid breakdown, decreasing acetyl-CoA for ketones, leading to low ketones despite hypoglycemia. This explains the episodic weakness and elevated acylcarnitines. Choice B is incorrect as it suggests excess ketogenesis, ignoring the oxidation block. For similar problems, focus on chain-length specificity and fuel deficits. Verify by checking for hypoketosis as a hallmark.

Question 2

A patient has progressive cardiomyopathy and skeletal muscle weakness. Enzyme assay shows deficiency of lysosomal acid ;1-glucosidase, with glycogen accumulation in lysosomes. Using the concept of subcellular localization of metabolic degradation, which outcome is most consistent?

  1. Elevated branched-chain ketoacids due to impaired mitochondrial dehydrogenase activity
  2. Impaired cytosolic glycolysis due to inability to convert glucose-6-phosphate to fructose-6-phosphate
  3. Increased hepatic ketone production due to excessive lysosomal glycogen breakdown
  4. Impaired lysosomal glycogen degradation leading to glycogen-filled lysosomes and muscle dysfunction (correct answer)
Explanation: This question tests lysosomal storage diseases from glucosidase mutations. The principle is that compartment-specific enzyme defects impair degradation, causing substrate-filled lysosomes and organ dysfunction. The acid alpha-glucosidase deficiency blocks lysosomal glycogen breakdown, leading to accumulation and muscle weakness. This explains cardiomyopathy and weakness in Pompe disease. Choice B is wrong because it confuses lysosomal with cytosolic pathways. In analogous scenarios, emphasize subcellular sites. Verify with biopsy accumulation patterns.

Question 3

In a newborn screening study, infants with poor feeding, vomiting, and lethargy were found to have elevated plasma leucine and its corresponding αα-ketoacid. Urine organic acids showed increased branched-chain ketoacids. Sequencing revealed a missense mutation in the E1 subunit of branched-chain αα-ketoacid dehydrogenase (BCKDH) that reduces catalytic activity but does not affect protein abundance. Based on the biochemical concept of an enzyme-catalyzed metabolic block, which outcome is most consistent with this mutation?

  1. Decreased levels of branched-chain amino acids due to increased transamination flux into the TCA cycle
  2. Accumulation of branched-chain amino acids and their αα-ketoacids upstream of the BCKDH step (correct answer)
  3. Increased conversion of branched-chain ketoacids back to amino acids by reversal of BCKDH
  4. Primary isolated hyperammonemia without elevation of branched-chain metabolites
Explanation: This question tests the understanding of inborn errors of metabolism due to mutations in catabolic enzymes, specifically branched-chain amino acid degradation. The relevant biochemical principle is that an enzyme deficiency creates a metabolic block, leading to accumulation of substrates immediately upstream and potential diversion into alternative pathways. In this case, the missense mutation in the E1 subunit of BCKDH reduces its activity, blocking the decarboxylation of branched-chain alpha-ketoacids derived from leucine, isoleucine, and valine. Therefore, branched-chain amino acids and their alpha-ketoacids accumulate upstream, as seen in the elevated plasma leucine and ketoacids, consistent with maple syrup urine disease. Choice A is incorrect because it suggests decreased branched-chain amino acids, which misconstrues the block as increasing flux rather than causing buildup. To approach similar questions, first identify the exact step blocked by the mutation. Then, map out the upstream metabolites that would accumulate and any toxic byproducts formed.

Question 4

A child with developmental regression has elevated very-long-chain fatty acids (VLCFAs) in plasma. A mutation is identified in a peroxisomal membrane transporter required to import activated VLCFAs for peroxisomal ββ-oxidation. Using the concept of compartment-specific fatty acid metabolism, which outcome is most consistent?

  1. Isolated hyperglycemia due to direct inhibition of insulin receptor signaling by VLCFAs
  2. Selective accumulation of short-chain fatty acids due to impaired mitochondrial carnitine shuttle
  3. Increased VLCFA oxidation in peroxisomes because transporters are not required for import
  4. Accumulation of VLCFAs due to impaired peroxisomal oxidation, with secondary effects on myelin stability (correct answer)
Explanation: This question examines peroxisomal disorders from transporter mutations. The principle is that compartment-specific defects prevent substrate entry, leading to accumulation and secondary tissue damage. The peroxisomal transporter mutation blocks VLCFA import for beta-oxidation, causing their plasma accumulation and myelin instability. This fits the developmental regression in X-linked adrenoleukodystrophy. Choice C is incorrect as it suggests increased oxidation without transporters, ignoring import necessity. For similar questions, emphasize organelle localization. Validate by noting elevated VLCFAs as diagnostic.

Question 5

A 6-week-old infant presents with poor feeding, vomiting, and lethargy after switching from breast milk to standard formula. Labs show metabolic acidosis and markedly elevated plasma leucine; urine organic acids reveal increased branched-chain αα-ketoacids. A genetic test identifies a missense mutation in the E1 subunit of branched-chain αα-ketoacid dehydrogenase (BCKDH) that lowers VmaxV_{max} without changing KmK_m for substrate. The biochemical concept is a mutation-caused enzyme defect producing a metabolic block and toxic intermediate accumulation.

Based on the scenario, which outcome is most consistent with the enzyme defect?

  1. Decreased production of acetyl-CoA from pyruvate with primary lactic acidosis due to impaired pyruvate dehydrogenase
  2. Accumulation of branched-chain amino acids and their αα-ketoacids with neurologic toxicity, worsened by high-protein intake (correct answer)
  3. Increased conversion of branched-chain αα-ketoacids back to branched-chain amino acids, preventing ketoacidosis
  4. Isolated hyperammonemia with normal organic acids due to failure of the urea cycle to incorporate nitrogen
Explanation: This question tests understanding of maple syrup urine disease (MSUD), an inborn error of branched-chain amino acid metabolism. MSUD results from deficiency of branched-chain α-ketoacid dehydrogenase (BCKDH), which normally catalyzes the oxidative decarboxylation of α-ketoacids derived from leucine, isoleucine, and valine. The mutation described reduces Vmax (maximum enzyme velocity) without changing Km, indicating decreased catalytic efficiency that causes accumulation of both branched-chain amino acids and their corresponding α-ketoacids upstream of the metabolic block. The correct answer B accurately describes this accumulation pattern and the neurologic toxicity that worsens with increased protein intake (providing more substrate). Answer A incorrectly describes pyruvate dehydrogenase deficiency rather than BCKDH deficiency, while C incorrectly suggests increased conversion back to amino acids would prevent ketoacidosis when actually the block prevents further metabolism. When analyzing enzyme defects, focus on what accumulates upstream of the block and what becomes deficient downstream.

Question 6

A toddler has recurrent kidney stones and hematuria. Urinalysis shows hexagonal crystals and elevated cystine. Genetic testing reveals a mutation in an apical renal transporter responsible for reabsorbing dibasic amino acids. Using the concept of loss of transport function leading to metabolite loss and precipitation, which outcome is most consistent?

  1. Elevated homogentisic acid due to impaired tyrosine catabolism
  2. Decreased urinary cystine due to increased transporter affinity for cystine
  3. Hyperammonemia due to impaired urea cycle transport of ornithine into mitochondria
  4. Increased urinary cystine due to impaired reabsorption, promoting cystine stone formation (correct answer)
Explanation: This question tests renal transport disorders from amino acid transporter mutations. The principle is that loss-of-function in reabsorptive transporters increases urinary excretion, leading to precipitation and stones. The dibasic amino acid transporter mutation impairs cystine reabsorption, elevating urinary cystine and forming stones. This accounts for the kidney stones and hexagonal crystals. Choice B is incorrect as it suggests decreased cystine from increased affinity, confusing loss-of-function with gain. For similar cases, link transport defects to solute loss. Verify with urinalysis findings like crystals.

Question 7

A newborn is screened and found to have elevated medium-chain acylcarnitines. The infant later develops hypoketotic hypoglycemia during a viral illness. Sequencing reveals a loss-of-function mutation in medium-chain acyl-CoA dehydrogenase (MCAD). The biochemical concept is an enzyme defect in ββ-oxidation that limits acetyl-CoA production and ketogenesis during fasting.

What physiological effect would be expected from the described genetic mutation?

  1. Increased ketone body production during fasting due to enhanced fatty acid oxidation
  2. Hypoketotic hypoglycemia during fasting because reduced ββ-oxidation decreases acetyl-CoA and ATP needed to support gluconeogenesis (correct answer)
  3. Isolated hyperammonemia after protein intake due to impaired carbamoyl phosphate synthesis
  4. Primary lactic acidosis at rest due to constitutive activation of pyruvate dehydrogenase kinase
Explanation: This question tests understanding of medium-chain acyl-CoA dehydrogenase (MCAD) deficiency, the most common fatty acid oxidation disorder. MCAD catalyzes the first dehydrogenation step in β-oxidation of C6-C12 fatty acids; its deficiency prevents efficient fatty acid oxidation during fasting, limiting acetyl-CoA production needed for both ketogenesis and gluconeogenesis energy requirements. The metabolic block causes accumulation of medium-chain acylcarnitines (diagnostic marker) and manifests as hypoketotic hypoglycemia during metabolic stress because inadequate β-oxidation cannot generate sufficient acetyl-CoA for ketone body synthesis or ATP for gluconeogenesis. The correct answer B accurately describes this hypoketotic hypoglycemia phenotype resulting from impaired β-oxidation. Answer A incorrectly suggests increased ketogenesis when it's actually impaired, while C and D describe unrelated metabolic defects. Recognition of fatty acid oxidation defects requires understanding the dual role of β-oxidation in providing both ketogenic substrate and energy for gluconeogenesis.

Question 8

In a metabolic study, fibroblasts from a patient show normal levels of lysosomal enzyme protein by Western blot, but markedly reduced enzymatic activity. The substrate of the enzyme accumulates in lysosomes, causing progressive neurodegeneration. Sequencing reveals a point mutation that changes a catalytic residue but does not alter trafficking signals. Using the concept of enzyme active-site mutations, which explanation best fits the data?

  1. The mutation increases enzyme affinity for substrate, accelerating substrate breakdown
  2. The mutation disrupts a catalytic residue, reducing kcatk_{cat} despite normal enzyme abundance and localization (correct answer)
  3. The mutation reverses the reaction direction, converting product back to substrate in lysosomes
  4. The mutation causes autosomal dominant inheritance because catalytic residues determine dominance
Explanation: This question examines the effects of active-site mutations in lysosomal storage disorders. The principle is that mutations altering catalytic residues impair enzyme function without affecting protein synthesis or localization, leading to substrate accumulation in compartments. The point mutation in the catalytic residue reduces the lysosomal enzyme's k_cat, causing substrate buildup despite normal protein levels and trafficking. This fits the neurodegeneration from lysosomal storage, as activity is low but abundance is normal. Choice A is incorrect because it implies increased affinity accelerating breakdown, confusing catalytic efficiency with substrate binding. To solve similar problems, distinguish between mutations affecting catalysis versus stability. Confirm by evaluating if symptoms align with compartment-specific accumulation.

Question 9

A 6-month-old presents with hepatomegaly, fasting hypoglycemia, and elevated blood lactate after brief fasting. Liver biopsy shows increased glycogen content. Enzyme assay demonstrates markedly reduced glucose-6-phosphatase activity in hepatocytes. Using the central concept of pathway bottlenecks caused by enzyme defects, which metabolic consequence is most consistent with this defect?

  1. Inability to release free glucose from the liver, causing accumulation of glucose-6-phosphate and increased glycolytic flux to lactate (correct answer)
  2. Impaired glycogen synthesis due to inability to form UDP-glucose, leading to low liver glycogen
  3. Increased hepatic ketogenesis due to excessive export of glucose into blood during fasting
  4. Reduced lactate production because glucose-6-phosphate cannot enter glycolysis
Explanation: This question assesses knowledge of glycogen storage diseases caused by mutations in gluconeogenic enzymes. The key principle is that defects in enzymes required for glucose release from the liver lead to intracellular trapping of glucose precursors, shunting them into alternative metabolic routes. Here, the glucose-6-phosphatase deficiency prevents dephosphorylation of glucose-6-phosphate, causing its accumulation in the liver and increased flux through glycolysis to lactate. This explains the hepatomegaly, hypoglycemia, and elevated lactate, as the liver cannot export glucose during fasting. Choice D is wrong because it assumes glucose-6-phosphate cannot enter glycolysis, ignoring that the defect is downstream, allowing glycolytic entry but blocking glucose release. For similar problems, locate the metabolic bottleneck and predict buildup of intermediates. Consider compensatory pathways like increased lactate production to confirm the diagnosis.

Question 10

A 9-month-old has failure to thrive, chronic diarrhea, and metabolic acidosis. Plasma shows elevated propionyl-CoA-derived metabolites and increased anion gap. Enzyme assay reveals reduced propionyl-CoA carboxylase activity. Based on the biochemical concept of anaplerotic entry into the TCA cycle, which consequence is most consistent?

  1. Increased urea production due to enhanced amino acid catabolism as the primary defect
  2. Increased succinyl-CoA formation from acetyl-CoA, causing TCA cycle overload
  3. Direct blockage of pyruvate dehydrogenase, leading to isolated lactic acidosis without organic acids
  4. Decreased conversion of propionyl-CoA to methylmalonyl-CoA, reducing succinyl-CoA replenishment and causing organic acid accumulation (correct answer)
Explanation: This question tests anaplerotic pathway defects from carboxylase mutations. The principle is that blocks in converting odd-chain precursors to TCA intermediates cause organic acid accumulation and metabolic acidosis. The propionyl-CoA carboxylase deficiency impairs conversion to methylmalonyl-CoA, reducing succinyl-CoA entry and building up propionyl-derived acids. This results in failure to thrive, diarrhea, and anion gap acidosis. Choice B is wrong because it claims increased succinyl-CoA from acetyl-CoA, confusing the pathway direction. In analogous cases, trace carbon flow to the TCA cycle. Confirm with elevated specific metabolites like propionate.

Question 11

A neonate presents with cataracts, vomiting after milk feeds, and jaundice. Labs show elevated galactose-1-phosphate in RBCs. Enzyme testing shows low galactose-1-phosphate uridyltransferase (GALT) activity. Considering the biochemical concept of toxic metabolite accumulation upstream of a metabolic block, which finding is most consistent?

  1. Decreased galactose-1-phosphate with increased UDP-galactose due to enhanced GALT activity
  2. Accumulation of galactose and galactose-1-phosphate with diversion of galactose to galactitol in the lens (correct answer)
  3. Increased conversion of galactose-1-phosphate to glucose-6-phosphate by reversal of GALT
  4. Isolated elevation of methylmalonic acid due to impaired odd-chain fatty acid oxidation
Explanation: This question tests knowledge of carbohydrate metabolism disorders from transferase mutations. The principle is that blocks in sugar conversion pathways cause accumulation of phosphorylated intermediates and reduction to polyols in tissues like the lens. The GALT deficiency prevents galactose-1-phosphate exchange to UDP-glucose, leading to galactose and its phosphate buildup, with aldose reductase converting galactose to galactitol causing cataracts. This accounts for the neonatal symptoms after milk ingestion. Choice C is incorrect as it proposes reversal of GALT, which is not possible due to thermodynamic barriers. For similar disorders, identify toxic accumulations and alternative pathways. Check consistency with dietary triggers like lactose.

Question 12

In a cohort study, some individuals with a mitochondrial enzyme mutation show symptoms only after exposure to a drug that increases oxidative stress. The mutation reduces activity of glucose-6-phosphate dehydrogenase (G6PD) in RBCs. The central concept is impaired NADPH generation affecting redox buffering. Which outcome is most consistent with this defect during oxidative stress?

  1. Primary elevation of phenylalanine due to impaired aromatic amino acid hydroxylation
  2. Increased NADPH production enhances detoxification of reactive oxygen species, preventing hemolysis
  3. Direct inhibition of hemoglobin synthesis causing microcytic anemia without hemolysis
  4. Reduced NADPH availability limits regeneration of reduced glutathione, increasing susceptibility to hemolysis (correct answer)
Explanation: This question evaluates redox metabolism defects from dehydrogenase mutations. The principle is that impaired NADPH production reduces glutathione regeneration, increasing oxidative damage susceptibility. The G6PD mutation limits NADPH in RBCs, impairing antioxidant defenses and causing hemolysis under stress. This explains drug-triggered symptoms in affected individuals. Choice B is wrong because it claims increased NADPH, misstating the deficiency effect. In comparable scenarios, connect NADPH to redox buffering. Confirm with triggers like oxidants for verification.

Question 13

A patient presents with photosensitivity and blistering skin lesions after sun exposure. Urine darkens on standing. Testing shows elevated uroporphyrinogen and reduced uroporphyrinogen decarboxylase activity. Considering the biochemical concept of porphyrin pathway enzyme defects causing intermediate buildup, which finding is most consistent?

  1. Primary neurologic crises due to acute GABA deficiency as the direct result of this enzyme defect
  2. Decreased porphyrins with increased heme synthesis, causing polycythemia
  3. Increased conversion of heme to porphyrins due to reversal of decarboxylase activity
  4. Accumulation of porphyrin intermediates that generate reactive species upon light exposure, contributing to cutaneous photosensitivity (correct answer)
Explanation: This question assesses porphyrias from decarboxylase mutations. The principle is that heme synthesis defects cause porphyrin intermediate buildup, leading to photosensitivity from reactive species generation. The uroporphyrinogen decarboxylase deficiency accumulates uroporphyrinogen, oxidizing to porphyrins that cause skin lesions upon light exposure. This explains the photosensitivity and urine darkening. Choice B is misleading because it claims decreased porphyrins with increased heme, reversing the block effect. In related problems, identify the porphyrin pathway step. Check for light-dependent symptoms to confirm.

Question 14

In a screening program, a child is found to have elevated methylmalonic acid and low succinyl-CoA formation from propionate. A mutation reduces methylmalonyl-CoA mutase activity. Using the concept of metabolic intermediate confusion in propionate metabolism, which outcome is most consistent?

  1. Primary hypoketotic hypoglycemia due to impaired medium-chain fatty acid oxidation
  2. Increased conversion of succinyl-CoA to methylmalonyl-CoA due to reversal of the mutase reaction
  3. Isolated elevation of orotic acid due to direct inhibition of OTC by methylmalonic acid
  4. Accumulation of methylmalonyl-CoA-derived metabolites with reduced entry of propionyl units into the TCA cycle as succinyl-CoA (correct answer)
Explanation: This question tests propionate metabolism disorders from mutase mutations. The principle is that blocks prevent TCA anaplerosis, accumulating upstream acids like methylmalonic. The methylmalonyl-CoA mutase deficiency reduces succinyl-CoA formation, building up methylmalonyl metabolites. This fits the elevated methylmalonic acid. Choice B is incorrect as it suggests reaction reversal, which is not thermodynamically favored. For similar problems, trace anaplerotic paths. Check metabolite patterns for confirmation.

Question 15

A 6-week-old infant presents with poor feeding, vomiting, and lethargy after switching from breast milk to formula. Physical exam shows hepatomegaly and jaundice. Labs: hypoglycemia and elevated ALT/AST. A newborn screen later reports markedly elevated blood galactose-1-phosphate. Sequencing identifies a homozygous missense mutation in the gene encoding galactose-1-phosphate uridyltransferase (GALT) that decreases catalytic activity but does not affect protein expression. The biochemical concept is an enzyme defect causing a metabolic block with upstream metabolite accumulation. Which of the following outcomes is most consistent with the enzyme defect described?

  1. Decreased conversion of galactose-1-phosphate to UDP-galactose, leading to accumulation of galactose-1-phosphate in hepatocytes (correct answer)
  2. Increased conversion of UDP-galactose to galactose-1-phosphate, causing depletion of UDP-glucose pools
  3. Impaired transport of galactose across the intestinal brush border, leading to osmotic diarrhea without hepatomegaly
  4. Reduced hepatic gluconeogenesis due to direct inhibition of fructose-1,6-bisphosphatase by galactose
Explanation: This question tests understanding of enzyme defects causing metabolic blocks with upstream metabolite accumulation. Galactose-1-phosphate uridyltransferase (GALT) catalyzes the conversion of galactose-1-phosphate to UDP-galactose in the Leloir pathway of galactose metabolism. When GALT activity is reduced due to a missense mutation, galactose-1-phosphate cannot be efficiently converted to UDP-galactose, causing it to accumulate in tissues, particularly hepatocytes. The correct answer (A) accurately describes this metabolic block and accumulation pattern, which explains the hepatomegaly and liver enzyme elevation seen in classic galactosemia. Choice B incorrectly reverses the reaction direction, as GALT catalyzes the forward reaction, not the reverse. To identify the correct answer in similar questions, focus on the directionality of the enzymatic reaction and recognize that enzyme defects cause substrate accumulation upstream of the block, not downstream product accumulation.

Question 16

During a fasting study, a child develops hypoketotic hypoglycemia. Plasma shows elevated medium-chain acylcarnitines. Genetic testing reveals a mutation that reduces medium-chain acyl-CoA dehydrogenase (MCAD) activity. The biochemical concept is impaired β\beta-oxidation limiting acetyl-CoA supply for ketogenesis and gluconeogenesis support. Which outcome is most consistent with this defect during prolonged fasting?

  1. Increased ketone body production due to shunting of fatty acids into mitochondrial oxidation
  2. Decreased acetyl-CoA generation reduces ketogenesis and limits activation of pyruvate carboxylase, worsening hypoglycemia (correct answer)
  3. Increased urea cycle activity causes hyperammonemia and elevated orotic acid after protein meals
  4. Defective glycogen branching enzyme causes hepatosplenomegaly and cirrhosis in early childhood
Explanation: This question tests understanding of medium-chain acyl-CoA dehydrogenase (MCAD) deficiency and the metabolic consequences of impaired β-oxidation. MCAD catalyzes the first dehydrogenation step in β-oxidation of medium-chain fatty acids (C6-C12), and its deficiency blocks fatty acid oxidation at this step, preventing acetyl-CoA generation from these substrates. During fasting, when fatty acid oxidation normally provides acetyl-CoA for ketogenesis and allosteric activation of pyruvate carboxylase (the rate-limiting enzyme of gluconeogenesis), MCAD deficiency limits both processes, resulting in hypoketotic hypoglycemia. The correct answer B accurately describes how decreased acetyl-CoA generation impairs both ketogenesis and gluconeogenesis support through reduced pyruvate carboxylase activation. Answer A is incorrect because it suggests increased ketone production, when actually ketogenesis is impaired due to insufficient acetyl-CoA substrate from blocked β-oxidation. To identify β-oxidation defects, look for hypoketotic hypoglycemia during fasting with elevated acylcarnitines corresponding to the chain length where the block occurs. Remember that acetyl-CoA from β-oxidation serves dual roles: substrate for ketogenesis and allosteric activator of gluconeogenesis.

Question 17

Researchers identify a homozygous missense mutation in hepatic fructose-1-phosphate aldolase (aldolase B) that reduces its VmaxV_{max} to 15% of normal. After ingestion of fruit juice, the patient develops vomiting, diaphoresis, and hypoglycemia. Lab tests shortly after symptoms show increased fructose-1-phosphate and decreased inorganic phosphate (Pi) in hepatocytes. The central biochemical concept is a metabolic block causing secondary depletion of a required cofactor/substrate pool. Which of the following best explains the hypoglycemia?

  1. Defective glucose-6-phosphatase prevents glycogen breakdown in muscle, causing rhabdomyolysis after exercise
  2. Reduced aldolase B activity directly increases pyruvate kinase flux, accelerating hepatic ATP production
  3. Fructose-1-phosphate is efficiently converted to glucose-6-phosphate, causing hyperglycemia
  4. Accumulated fructose-1-phosphate traps Pi, decreasing ATP and impairing gluconeogenesis and glycogenolysis (correct answer)
Explanation: This question tests understanding of hereditary fructose intolerance and the concept of metabolic trapping. Aldolase B catalyzes the cleavage of fructose-1-phosphate to dihydroxyacetone phosphate and glyceraldehyde in the liver, and its deficiency causes accumulation of fructose-1-phosphate after fructose ingestion. The accumulated fructose-1-phosphate sequesters inorganic phosphate (Pi), depleting the free Pi pool needed for ATP synthesis and as a substrate for glucose-6-phosphatase in gluconeogenesis and glycogenolysis, resulting in hypoglycemia. The correct answer D accurately describes this Pi trapping mechanism and its consequences on hepatic glucose production. Answer B is incorrect because it suggests increased ATP production, when actually ATP is depleted due to Pi sequestration and impaired oxidative phosphorylation. When evaluating metabolic blocks, consider secondary effects on cofactor pools (Pi, NAD+, CoA) that can explain seemingly unrelated symptoms. The key concept is that substrate accumulation can deplete essential cofactors, causing dysfunction in other pathways.

Question 18

In an experiment, hepatocytes expressing a mutant glucose-6-phosphatase show normal glycogen synthase activity but cannot release free glucose into the bloodstream. After an overnight fast, the model organism has hepatomegaly, fasting hypoglycemia, and elevated blood lactate. The central concept is a metabolic block at the final step of gluconeogenesis and glycogenolysis in liver. Which of the following best explains the elevated lactate?

  1. Impaired fructose-1,6-bisphosphatase increases ketone production and causes hyperglycemia after fasting
  2. Increased hepatic export of glucose reduces peripheral glycolysis, causing lactate accumulation in blood
  3. Defective pyruvate dehydrogenase prevents acetyl-CoA formation, causing isolated lactic acidosis without hypoglycemia
  4. Blocked conversion of glucose-6-phosphate to glucose increases diversion of glucose-6-phosphate into glycolysis, raising lactate (correct answer)
Explanation: This question tests understanding of von Gierke disease (GSD type Ia) and the metabolic consequences of glucose-6-phosphatase deficiency. Glucose-6-phosphatase catalyzes the final step of both gluconeogenesis and glycogenolysis in liver, converting glucose-6-phosphate to free glucose for export; its deficiency traps glucose-6-phosphate in hepatocytes. The accumulated glucose-6-phosphate cannot exit the cell and is diverted into glycolysis, producing pyruvate that is converted to lactate by lactate dehydrogenase to regenerate NAD+ for continued glycolysis, leading to lactic acidosis even during fasting when gluconeogenesis should predominate. The correct answer D accurately describes how blocked glucose export increases diversion into glycolysis and lactate production. Answer B is incorrect because it suggests increased glucose export, when the mutation specifically prevents glucose release from hepatocytes. To understand glucose-6-phosphatase deficiency, remember it affects the final common pathway of hepatic glucose production, causing fasting hypoglycemia, lactic acidosis, hyperuricemia, and hepatomegaly from glycogen accumulation. The key concept is that metabolic blocks force substrate diversion into alternative pathways.

Question 19

A child has recurrent infections and failure to thrive. Neutrophils show absent NADPH oxidase activity. Genetic testing reveals a mutation that prevents assembly of the membrane-bound oxidase complex. The biochemical concept is how defects in NADPH-dependent pathways affect cellular function. Which finding is most consistent with this mutation?

  1. Inability to generate reactive oxygen species in phagolysosomes, leading to impaired killing of catalase-positive organisms (correct answer)
  2. Increased oxidative burst with excessive hypochlorite formation, causing severe tissue necrosis after minor infections
  3. Defective urea cycle enzyme causing elevated ammonia and increased orotic acid in urine
  4. Reduced synthesis of heme due to impaired ALA synthase, causing microcytic anemia and basophilic stippling
Explanation: This question tests understanding of chronic granulomatous disease and the role of NADPH oxidase in phagocyte function. NADPH oxidase is a multi-subunit enzyme complex that generates superoxide radicals from molecular oxygen using NADPH as an electron donor, initiating the respiratory burst that produces reactive oxygen species (ROS) for microbial killing. When NADPH oxidase cannot assemble properly, phagocytes cannot generate superoxide and downstream ROS like hydrogen peroxide and hypochlorite, making them unable to kill catalase-positive organisms that can neutralize any residual hydrogen peroxide from other sources. The correct answer A accurately describes the inability to generate ROS and the specific susceptibility to catalase-positive organisms like Staphylococcus aureus. Answer B is incorrect because it suggests increased oxidative burst, when the mutation actually prevents ROS generation entirely. To identify phagocyte defects, remember that chronic granulomatous disease specifically impairs the oxidative burst, while myeloperoxidase deficiency affects only hypochlorite production but maintains some antimicrobial activity. The key diagnostic test is the nitroblue tetrazolium (NBT) or dihydrorhodamine (DHR) test showing absent oxidative burst.

Question 20

A newborn develops poor feeding and lethargy 36 hours after birth. Plasma ammonia is 210 μ\mumol/L (elevated), and plasma orotic acid is elevated. Liver biopsy shows normal carbamoyl phosphate synthetase I activity but markedly reduced ornithine transcarbamylase (OTC) activity. The mutation decreases OTC catalytic efficiency without changing protein abundance. Based on this enzyme defect, which outcome is most likely and most consistent with the biochemical findings?

  1. Decreased cytosolic carbamoyl phosphate causes reduced pyrimidine synthesis and low orotic acid
  2. Accumulation of mitochondrial carbamoyl phosphate increases cytosolic orotic acid production via pyrimidine synthesis (correct answer)
  3. Increased conversion of ornithine to citrulline lowers ammonia and prevents neurologic symptoms
  4. Primary impairment of phenylalanine hydroxylase leads to elevated phenylketones and musty odor
Explanation: This question tests understanding of urea cycle enzyme deficiencies and their metabolic consequences. Ornithine transcarbamylase (OTC) catalyzes the conversion of ornithine and carbamoyl phosphate to citrulline in the mitochondria, and its deficiency causes the most common urea cycle disorder. When OTC activity is reduced, carbamoyl phosphate accumulates in the mitochondria and leaks into the cytosol, where it enters the pyrimidine synthesis pathway via carbamoyl phosphate synthetase II, leading to increased orotic acid production. The correct answer B accurately describes this pathophysiology, explaining both the elevated ammonia (due to impaired urea cycle) and elevated orotic acid (due to shunting into pyrimidine synthesis). Answer A is incorrect because it suggests decreased cytosolic carbamoyl phosphate and low orotic acid, which is opposite to what occurs in OTC deficiency. To identify urea cycle defects, remember that OTC deficiency uniquely causes both hyperammonemia and orotic aciduria, while carbamoyl phosphate synthetase I deficiency causes hyperammonemia without orotic aciduria.