MCAT Biology Question 127: Answer and Explanation

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Question: 127

7. A two-week-old male infant is brought to the emergency room. His mother reports that he has been unable to keep any milk down; shortly after he nurses, he has sudden projectile vomiting. During exam, an olive-shaped mass can be felt in his upper abdomen. It is determined that there is a constriction in the digestive system that prevents food from reaching the small intestine from the stomach. Which structure is most likely the site of the problem?

  • A. Cardiac sphincter
  • B. Pyloric sphincter
  • C. Ileocecal valve
  • D. Internal anal sphincter

Correct Answer: B

Explanation:

The question is basically asking us to identify the structure that lies between the stomach and the small intestine. This is the pyloric sphincter; the presentation given in the question is a classic example of what is called pyloric stenosis, in which the pyloric sphincter is thickened and cannot relax to permit chyme through. The cardiac sphincter, choice (A), lies between the esophagus and the stomach. The ileocecal valve, choice (C), lies between the ileum of the small intestine and the cecum of the large intestine. The internal anal sphincter, choice (D), lies at the end of the rectum.

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