The defaecation reflex:

Correct Answer: C

Rationale: The correct answer is C because the defecation reflex is integrated in a control center in the sacral spinal cord. This is due to the presence of the parasympathetic nerves that control the smooth muscles of the colon and rectum in the sacral region. The sacral spinal cord is responsible for coordinating the reflexive relaxation of the internal anal sphincter and contraction of the rectum during defecation. Choices A and B are incorrect because the initiation and integration of the defecation reflex do not involve the anal canal stretching or the lumbar spinal cord. Choice D is incorrect as the defecation reflex is not controlled by the vagus nerve, which is primarily responsible for the parasympathetic innervation of organs in the thorax and upper abdomen.

Correct Answer: D

Rationale: The correct answer is D because G cells in the gastric pit secrete gastrin, not histamine. Gastrin stimulates the release of hydrochloric acid in the stomach. Mucous cells are not enteroendocrine, so A is incorrect. G cells are not exocrine, making B incorrect. D cells secrete somatostatin, not histamine, so C is also incorrect.

Correct Answer: C

Rationale: The correct answer is C because carcinoid syndrome can occur with carcinoid tumors of the appendix and ileum. Carcinoid tumors of the appendix can lead to carcinoid syndrome due to the release of serotonin and other vasoactive substances. Similarly, carcinoid tumors localized to the ileum can also produce these substances, leading to the development of carcinoid syndrome. Choices A and B alone are not comprehensive enough as both sites can give rise to carcinoid syndrome. Choice D is incorrect as carcinoid syndrome can indeed occur with tumors from both sites.

Correct Answer: A

Rationale: The correct answer is A. Mass peristalsis refers to the coordinated contraction of the smooth muscle in the colon that helps move fecal matter towards the rectum. This process specifically occurs in the large intestine, not the small intestine. Therefore, choice A is incorrect. Choice B is incorrect because mass peristalsis is actually a rapid and powerful movement, not slow. Choice C is correct as it accurately describes the role of mass peristalsis in facilitating the movement of fecal matter in the large intestine. Choice D is incorrect because choice A is incorrect.

Correct Answer: B

Rationale: The correct answer is B because swallowing is controlled by the swallowing center in the medulla oblongata. This center coordinates the complex sequence of muscle contractions involved in swallowing, ensuring food moves from the mouth to the esophagus without entering the airway. The pharyngeal phase is actually involuntary, not voluntary (Choice A). Swallowing usually occurs with a closed mouth, not an open one (Choice C). Additionally, the buccal phase is not accompanied by apnea; breathing typically continues during this phase (Choice D). Therefore, the correct answer is B due to the central control of swallowing in the medulla oblongata.