A 50-year-old man with prostate cancer received radiation to the prostate and now has xerostomia. Considering this history, which is the LEAST likely cause of his dry mouth?

Correct answer: A. His radiation therapy (it was to the prostate, far from salivary glands)

• A. Correct. Radiation causes xerostomia only when it targets the head/neck salivary glands; prostate (pelvic) radiation does not reach them, so it is the least likely cause here.
• B. Many systemic medications cause xerostomia and are plausible contributors.
• C. Dehydration is a common, plausible cause of dry mouth.
• D. Reduced salivary reserve with age can contribute.

Note: This is the trick: radiation is a classic xerostomia cause, but only head/neck radiation — not the prostate radiation this patient received.