What is the most likely reason for diarrhea in a patient who believes his antacids are the cause?

The most likely reason for diarrhea in a patient who believes his antacids are the cause pertains to the presence of magnesium in certain antacids. Magnesium-based antacids, such as magnesium hydroxide, can have a laxative effect when taken in larger doses. This is due to magnesium's ability to increase osmotic pressure in the intestines, leading to an increase in water content within the bowel and facilitating bowel movements. Consequently, patients using these antacids might experience diarrhea as a side effect.

The other options provided are less likely to be responsible for causing diarrhea. For example, while some sugar alcohols like sorbitol and mannitol can have a laxative effect, they are not commonly found in all antacids. Additionally, caffeine is not a typical ingredient in antacids, and fructose is not relevant in this context as antacids generally do not contain significant amounts of it. Thus, magnesium stands out as the most plausible explanation for the patient's experience with diarrhea related to his antacid use.