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Two substances are isolated from the body. When administered at the same time, these substances induce gastric acid secretion that is four times as great as that induced by either one alone. The two substances are most likely to be which of the following?

(A) Acetylcholine and secretin
(B) Gastrin and cholecystokinin
(C) Gastrin and histamine
(D) Histamine and vasoactive intestinal polypeptide
(E) Secretin and cholecystokinin
 

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This question examines the concept of "synergistic effect" (two substances/hormones/drugs, when combined together, have a net effect greater than the one expected by simple addition of these substances).

It is also required to recall the effect of various hormones onto gastric acid secretion.

To remember that gastrin increases gastric acid secretion (as its name indicates), have in mind the case of recalcitrant gastric ulcer in the context of gastrinoma.

To remember that histamine increases gastric acid secretion, recall the usefulness of H2 antagonists (ranitidine, famotidine & other -tidines) in the symptomatic relief of epigastric pain.

So the correct answer is C.

Also remember that Ach generally increases GI tract secretions & motility, and thus it increases gastric acid secretion (that's the usefulness of selective vagotomy in discrete cases of non-responding gastric ulcers).
 
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