Which cancer is indicated by a positive stool exam for occult blood?

A positive stool exam for occult blood is most commonly associated with colorectal cancer. This test is designed to detect hidden blood in the stool, which can be an early indicator of abnormalities in the gastrointestinal tract, including colorectal cancer.

Colorectal cancer often causes bleeding as tumors grow and invade surrounding tissues, leading to the presence of blood that may not be visible to the naked eye. The detection of occult blood is therefore a significant screening tool that prompts further investigation, such as colonoscopy, to identify and diagnose any potential malignancies within the colon or rectum.

In contrast, the other conditions listed do not typically present with a positive stool exam for occult blood. Diarrhea is a symptom and doesn't inherently indicate any specific malignancy. Achalasia, a disorder related to the esophagus, and aphthous stomatitis, which refers to mouth ulcers, do not produce occult blood in stool and are unrelated to colorectal cancer screening. Thus, the association of occult blood in stool tests is specifically tied to the risk of colorectal cancer.