**Core Concept**
Mucositis is a common side effect of chemotherapy, characterized by inflammation and ulceration of the mucous membranes, particularly in the mouth and gastrointestinal tract. It is primarily associated with drugs that interfere with DNA synthesis or repair, especially those that inhibit thymidylate synthase or disrupt rapidly dividing cells.

**Why the Correct Answer is Right**
5-Fluorouracil (5-FU) is a thymidylate synthase inhibitor that interferes with DNA synthesis by blocking thymidine production. This leads to inhibition of rapidly dividing cells, including those in the mucosal lining of the gastrointestinal tract and oral cavity. As a result, mucositis is a well-documented and common adverse effect of 5-FU therapy, especially at higher doses or prolonged exposure.

**Why Each Wrong Option is Incorrect**
Option B: Methotrexate, while also a DNA synthesis inhibitor, primarily affects folate metabolism and is more commonly associated with myelosuppression and gastrointestinal toxicity, but mucositis is less frequent and less severe than with 5-FU.
Option C: Paclitaxel acts by stabilizing microtubules and is more commonly linked to neurotoxicity and hypersensitivity reactions, not mucositis.
Option D: Cisplatin causes nephrotoxicity and neurotoxicity, and while it can cause gastrointestinal irritation, mucositis is not its hallmark toxicity.

**Clinical Pearl / High-Yield Fact**
5-FU is the most frequently reported cause of chemotherapy-induced mucositis among antineoplastic agents. Recognizing this helps in early prophylaxis and management—such as hydration, oral care, and nutritional support—especially in patients undergoing fluorouracil-based regimens.

✓ Correct Answer: A. 5-Fluorouracil