The small intestine is divided into three subsections in the following order: duodenum, jejunum, and ileum. The duodenum is responsible for neutralizing the acidity of the chyme entering the intestine from the stomach and introducing digestive enzymes. The jejunum and ileum are responsible for absorbing nutrients.

The small intestine is essential for the chemical digestion of nutrients and for their absorption. Pancreatic digestive enzymes are secreted into the duodenum, where they catabolize nutrients. These nutrients are then absorbed in the jejunum and ileum. Proteins and carbohydrates are absorbed directly into the blood, while fats are absorbed into the lymphatic lacteals.

Water, however, is not absorbed in large quantities in the small intestine. Instead, most water absorption occurs in the large intestine.

The lower esophageal sphincter separates the esophagus from the stomach. When stomach acid and contents leak through the lower esophageal sphincter, the acid irritates the esophagus and can lead to pain around the area of the heart. The heart is not actually affected.

The pyloric sphincter is found between the stomach and the duodenum of the small intestine. The other 3 sphincters are found in different parts of the body. 

Food enters the stomach from the esophagus through the cardiac sphincter.

The pyloric sphincter connects the stomach to the duodenum. The ileocecal sphincter connects the ileum to the large intestine, while the internal (anal) sphincter controls the exit of fecal matter from the body. There is also an external anal sphincter, which is under voluntary control, while the internal is involuntary.  

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.