The esophageal phase of swallowing is caused by the

Correct Answer: A

Rationale: The correct answer is A: peristaltic reflex. During the esophageal phase of swallowing, peristalsis is the coordinated contraction and relaxation of muscles that propels food down the esophagus. This reflex is initiated by the stretching of the esophageal wall when food bolus enters, triggering a series of contractions to move the bolus towards the stomach. The enteric reflex (B) involves local reflexes within the gut but is not specific to the esophageal phase of swallowing. The swallowing center (C) in the medulla oblongata coordinates the overall swallowing process but does not directly cause the esophageal phase. The coughing reflex (D) is a protective mechanism to clear the airway, not involved in the esophageal phase.

Correct Answer: B

Rationale: The correct answer is B because the pancreatic islets, also known as the islets of Langerhans, are responsible for producing hormones such as insulin, glucagon, and somatostatin. Insulin helps lower blood sugar levels, glucagon raises blood sugar levels, and somatostatin regulates the release of both insulin and glucagon. The other choices are incorrect because mucus is produced by goblet cells in the intestines, enterokinase is produced by the duodenum to activate pancreatic enzymes, and digestive enzymes are produced by the exocrine cells in the pancreas, not the pancreatic islets.

Correct Answer: C

Rationale: The correct answer is C: pancreas. The pancreas is the main source of lipase production in the body. Lipase is an enzyme responsible for breaking down fats into fatty acids and glycerol. The pancreas secretes pancreatic lipase into the small intestine where it aids in the digestion of fats. Salivary glands produce amylase for carbohydrate digestion, the stomach produces gastric lipase for limited fat digestion, and the small intestine produces additional digestive enzymes but not the majority of lipase.

Correct Answer: C

Rationale: Rationale for Correct Answer C: Enteropeptidase converts trypsinogen to trypsin by cleaving a peptide sequence that blocks the active site of trypsin. This is crucial for the activation of trypsin, a key enzyme in protein digestion. Trypsin then activates other pancreatic enzymes. This process ensures efficient breakdown of proteins in the small intestine. Summary of Incorrect Choices: A: Pepsinogen is not secreted by the pancreas and does not require removal of a peptide sequence from the active site for activation. B: Pancreatic α-amylase is responsible for breaking down starch into maltose and not glucose. D: Pancreatic secretion of hydrochloric acid is not directly related to the activation of pancreatic enzymes; it is the role of stomach acid to provide an acidic environment for pepsin activation.

Correct Answer: C

Rationale: The correct answer is C because the formation of tripartite synapses with astrocytes and neurons does not provide direct protection to the brain. Tripartite synapses involve communication between neurons and astrocytes, primarily for synaptic regulation and plasticity. The blood-brain barrier (A) physically protects the brain by restricting the passage of harmful substances. Microglia (B) function as the brain's immune cells, defending against pathogens and removing cellular debris. Therefore, choices A and B contribute to the brain's protection, making them incorrect answers.