Which substances inhibit gastric acid secretion?

• A. Gastrin and histamine inhibit gastric acid secretion.
• B. Acetylcholine inhibits gastric acid secretion.
• C. Cholecystokinin inhibits gastric acid secretion.
• D. Somatostatin and secretin inhibit gastric acid secretion.

Answer: (D)

Gastric acid secretion is controlled by a balance of signals—some stimulate it, others inhibit it. The substances that inhibit acid output do so by dampening the main drivers of acid production: gastrin release, histamine’s action on parietal cells, and direct parietal cell activity.

Somatostatin, produced by D cells in the stomach, is a broad brake on acid secretion. It reduces gastrin release from G cells and also directly curbs parietal cell activity. Secretin, released from the small intestine in response to acidic chyme, further lowers acid output by inhibiting gastrin release and decreasing parietal cell responsiveness, while also promoting bicarbonate secretion to neutralize acid.

The other options involve stimulatory factors or hormones not primarily aimed at inhibiting acid. Gastrin and histamine promote acid secretion; acetylcholine from vagal stimulation increases it; cholecystokinin mainly slows gastric processes and digestion rather than serving as a primary inhibitor of acid. So somatostatin and secretin best fit as inhibitors of gastric acid secretion.