The following is true about the small intestine:

Correct Answer: D

Rationale: The correct answer is D: The small intestine has segmentation contractions. This is because segmentation contractions are responsible for mixing and moving chyme (partially digested food) along the small intestine, allowing for optimal nutrient absorption. This process helps maximize the contact between chyme and the intestinal walls, facilitating absorption. A: Incorrect. The small intestine does not store food for several hours; instead, it continuously processes and absorbs nutrients. B: Incorrect. While the small intestine plays a crucial role in digestion and absorption, the main source of digestive enzymes is the pancreas. C: Incorrect. Haustrae are found in the large intestine, not the small intestine. They aid in the absorption of water and electrolytes.

Correct Answer: A

Rationale: The correct answer is A. Peristalsis involves coordinated waves of muscle contractions that propel food forward. However, peristalsis can also involve backward movements to help with mixing and digestion. Therefore, the statement that peristalsis only involves forward movements is incorrect. Segmentation involves mixing and breaking down food through contractions that occur in non-unidirectional patterns. Peristalsis can be found in the esophagus, stomach, and intestines, while segmentation is mainly present in the small intestine. Therefore, choices B, C, and D are incorrect based on the explanations provided.

Correct Answer: C

Rationale: Infarction of the bowel may be caused by a volvulus, where the bowel twists on its mesentery, leading to compromised blood flow. Mesenteric artery thrombosis can also cause bowel infarction by obstructing blood supply. Therefore, the correct answer is C (both). Choices A and B are incorrect because each of them individually can cause bowel infarction.

Correct Answer: B

Rationale: The correct answer is B: Sodium-glucose co-transport system. This system involves the active transport of glucose across the small intestine epithelial cells with the help of sodium ions. This mechanism allows for the absorption of glucose against its concentration gradient, ensuring efficient uptake. Other choices are incorrect because: A) Simple passive diffusion is not efficient for glucose absorption due to its hydrophilic nature. C) Osmosis is the movement of water molecules, not glucose. D) Primary active transport involves the direct use of energy to transport molecules, which is not the case for glucose absorption in the small intestine.

Correct Answer: B

Rationale: Step-by-step rationale: 1. Gastric secretions contain intrinsic factor, essential for vitamin B12 absorption from the terminal ileum. 2. Intrinsic factor binds to vitamin B12 to facilitate its absorption. 3. Without intrinsic factor, vitamin B12 absorption is impaired, leading to pernicious anemia. 4. Therefore, choice B is correct. Summary: - Choice A is incorrect as gastric secretions increase, not decrease, when a person thinks of food. - Choice C is incorrect as hydrochloric acid (HCl) is secreted by parietal cells, not chief cells. - Choice D is incorrect as fat digestion primarily occurs in the small intestine with the help of pancreatic enzymes, not gastric secretions.