The lower esophageal sphincter (LES) allows food to enter the stomach from the lower esophagus. The LES is under autonomic control, meaning that once food enters the esophagus, it is propelled via peristaltic waves towards the stomach through the lower esophageal sphincter. The pyloric sphincter is between the antrum of the stomach and the duodenum of the small intestine, while the ileocolic sphincter is between the ileum and the colon.

The parasympathetic nervous system activates parietal cells to secrete hydrochloric acid via the vagus nerve. Prolonged parasympathetic nervous system activation will result in a very high concentration of acid in the stomach. At an extremely low pH, the parietal cells can be damaged because the mucous that normally lines the stomach will be degraded. The destruction of parietal cells may will lower intrinsic factor secretion and lead to a decrease in vitamin B12 absorption. On the other hand a very low stomach pH will further break down macromolecules and destroy pathogens, but again, the body strives to maintain homeostasis among all parameters.

Lisa's anemia could be due to the inability to absorb vitamin B12. Without vitamin B12, Lisa is not able to produce red blood cells. Intrinsic factor secreted by parietal cells is essential for absorption. Down the gastrointestinal tract, vitamin B12 is absorbed by the ileum (distal small intestine). In patients with Helicobacter pylori infections, the pathogen causes destruction of the parietal cell and results in their inability to secrete intrinsic factor.  

The correct answer is:

This question tests the basic principles of what happens to the acidity of food throughout the digestive tract. In this example, the food began with a pH of 7.0 when the subject ate it. The question then tests your knowledge of what happens to the pH of food after digestion in the stomach and small intestine. 

The stomach acidifies food, as parietal cells produce H+ that enters in the stomach. Therefore, we would expect the pH of the food to be lower than when it was first consumed. In this case, 3.0 is the most appropriate listed pH for post-stomach, pre-small intestine digestion. 

When food enters the small intestine from the stomach, the small intestine alkalinizes the acidic stomach contents, causing the pH of the food to rise above what it was in the stomach. In this case, if it is leaving the stomach with a pH of 3.0, a pH of 6.0 after small intestinal digestion is reasonable. 

None of the other answer choices accurately reflect that food is acidified in the stomach and alkalinized in the small intestine. 

Bile is released from the gall bladder into the small intestine, where it helps with the emulsification of fats. Bile salts and phospholipids are crucial amphipathic compounds that allow lipids to associate while in the aqueous environment of the small intestine. Cholesterol promotes fluidity and prevents the lipids from becoming tightly packed. Bilirubin is a byproduct produced in the liver and secreted in bile; it is later reabsorbed in the ileum and returned to the liver in a cyclic pattern.

Proteases are secreted from the pancreas and assist in protein cleavage in the small intestine. They are not involved in bile or lipid digestion.

The correct answer is parietal cells. Parietal cells are responsible for HCl secretion in the stomach, which lowers the overall pH of the stomach. Chief cells secrete pepsinogen, which is converted to pepsin and is responsible for digesting proteins. G cells secrete gastrin, which stimulates parietal cells.

Only one of the enzymes listed functions in the stomach, where high acidity results in a low pH. That enzyme is pepsin. Other enzymes listed function in the small intestine, and will be denatured by high acidity.

Digestion of proteins begins in the stomach. The low pH of the stomach converts pepsinogen into pepsin, which is then used to break down proteins.

In the stomach, food is broken down using a host of different cells and enzymatic processes. Chief cells release pepsinogen, making it the correct answer. Parietel cells, which release HCl, allow the conversion of pepsinogen (a zymogen) into pepsin. Pepsin actively digests proteins in the highly acidic environment of the stomach.

Goblet cells release mucus which lines the stomach for protection.

In order to stimulate digestion in the stomach G cells secrete gastrin, which stimulates parietal cells. Pepsinogen is released by chief cells. The parietal cells release hydrochloric acid into the stomach lumen. The lowered acidity cleaves pepsinogen and creates pepsin, which begins to degrade proteins. 

Mucous cells have the nondigestive role of lubricating the stomach lumen and protecting the stomach epithelium from degradation by the highly acidic gastric juices.