Which statement best describes the stool effects of antacids based on their salt form?

• A. Magnesium-containing antacids cause constipation
• B. Calcium-containing antacids cause diarrhea
• C. Magnesium-containing antacids can cause diarrhea and calcium-containing antacids can cause constipation
• D. Aluminum-containing antacids cause diarrhea

Answer: (C)

Stool effects of antacids depend on the salt form. Magnesium salts act as osmotic laxatives in the gut, drawing water into the intestinal lumen and increasing bowel motions, which can lead to diarrhea. Calcium salts, in contrast, tend to slow intestinal transit and can cause harder stools or constipation. Aluminum-containing antacids also slow bowel movements and can contribute to constipation.

Thus, the statement that magnesium-containing antacids can cause diarrhea and calcium-containing antacids can cause constipation captures the opposite directions of their typical stool effects based on salt form. The other options misstate these relationships: magnesium does not usually cause constipation, calcium does not usually cause diarrhea, and aluminum-containing antacids are not a common cause of diarrhea.