A patient undergoing head and neck chemotherapy presents with dry mouth and a chief complaint of oral pain and painful swallowing. What is the MOST likely diagnosis?

Correct answer: B. Oral mucositis

• A. It is common, but it is not benign or ignorable — severe mucositis interrupts cancer therapy and is a major cause of hospitalization.
• B. CORRECT. Chemotherapy-induced oral MUCOSITIS results from direct cytotoxic injury to the rapidly dividing basal keratinocytes of the mucosa, producing diffuse erythema then painful ulceration of NON-KERATINIZED surfaces (buccal mucosa, ventral tongue, soft palate). It peaks around 7–14 days after treatment and causes odynophagia (painful swallowing) severe enough to require nutritional support and opioid analgesia.
• C. Candidiasis is a frequent superimposed infection, but it presents as WIPEABLE white plaques; the pain and odynophagia described are the mucositis itself.
• D. Primary herpetic gingivostomatitis presents with fever, lymphadenopathy, and involvement of KERATINIZED tissue and gingiva, typically in young children.

Note: Mucositis = non-keratinized tissue; herpes = keratinized tissue. That single distinction resolves most exam items.