MCAT - Organ Systems:: Digestive System

Salivary amylase begins carbohydrate digestion in the mouth. Which of the following best describes its subsequent fate and activity once a food bolus reaches the stomach?

- **A)** It continues to hydrolyze starch indefinitely because gastric pH has no effect on it
- **B)** It is progressively inactivated as gastric HCl lowers the pH below its optimum
- **C)** It is converted into pepsin by gastric chief cells
- **D)** It is reabsorbed intact into the portal circulation for reuse in the small intestine

Pepsinogen is secreted by gastric chief cells as an inactive zymogen. Which mechanism correctly describes its activation?

- **A)** Enterokinase in the duodenum cleaves pepsinogen to pepsin
- **B)** Gastric HCl lowers pH enough to cause autocatalytic cleavage of pepsinogen to pepsin
- **C)** Bile salts emulsify pepsinogen, exposing its active site
- **D)** Gastrin directly phosphorylates pepsinogen to activate it

Intrinsic factor is essential for normal digestive physiology. Loss of parietal cell mass (e.g., in atrophic gastritis) would most directly impair which downstream process?

- **A)** Absorption of vitamin B12 in the terminal ileum
- **B)** Absorption of dietary iron in the duodenum
- **C)** Emulsification of dietary lipids in the small intestine
- **D)** Activation of pancreatic lipase

Secretin is released from S cells of the duodenum in response to acidic chyme. Which of the following is its primary physiological effect?

- **A)** Stimulates gallbladder contraction to release stored bile
- **B)** Stimulates pancreatic ductal cells to secrete a bicarbonate-rich fluid
- **C)** Stimulates gastric chief cells to increase pepsinogen output
- **D)** Inhibits pancreatic acinar cell enzyme secretion

Cholecystokinin (CCK) is released in response to fatty acids and amino acids in the duodenum. Which combination of effects is most characteristic of CCK?

- **A)** Gallbladder contraction and relaxation of the sphincter of Oddi, plus stimulation of pancreatic enzyme secretion
- **B)** Inhibition of gallbladder contraction and stimulation of gastric acid secretion
- **C)** Stimulation of salivary amylase release and inhibition of pancreatic bicarbonate
- **D)** Contraction of the lower esophageal sphincter and inhibition of gastric emptying only

Gastrin is secreted by G cells of the antrum in response to gastric distension and protein digestion products. Its principal downstream effect is best described as:

- **A)** Stimulation of parietal cells to secrete HCl (partly via histamine release from enterochromaffin-like cells)
- **B)** Direct inhibition of pepsinogen secretion
- **C)** Stimulation of pancreatic bicarbonate secretion exclusively
- **D)** Contraction of the lower esophageal sphincter to prevent reflux

Bile salts are amphipathic molecules that emulsify dietary lipids. Which property allows them to perform this function?

- **A)** They are purely hydrophobic, allowing them to dissolve entirely within lipid droplets
- **B)** They have both hydrophilic and hydrophobic faces, letting them coat lipid droplets and reduce surface tension so mechanical agitation can break large droplets into smaller ones
- **C)** They act as covalent catalysts that chemically bond to triglycerides
- **D)** They lower the pH of the intestinal lumen, denaturing lipid droplets

After lipolysis, fatty acids and monoglycerides are packaged with bile salts into mixed micelles. What is the functional purpose of micelle formation?

- **A)** To chemically digest fatty acids into glycerol and CO2
- **B)** To transport otherwise water-insoluble lipid digestion products across the unstirred water layer to the enterocyte brush border for absorption
- **C)** To transport lipids directly into the hepatic portal vein
- **D)** To activate pancreatic lipase at the intestinal brush border