A nurse is caring for a client who has a prescription for a stool test for guaiac. The nurse understands the purpose of the test is to check the stool for which of the following substances?

The correct answer is D: Blood. Guaiac test is used to detect occult (hidden) blood in the stool, which may indicate various gastrointestinal conditions like bleeding ulcers, colorectal cancer, or inflammatory bowel disease. The test involves applying guaiac reagent to a stool sample, and a blue color change indicates the presence of blood. Parasites (A), bacteria (B), and steatorrhea (C) are not substances typically tested for using the guaiac test. Parasites are usually identified through specific stool tests, bacteria through stool cultures, and steatorrhea through fecal fat tests.