**Question:** Which of the following is not an early adverse effect of methotrexate?

A. Mouth ulcers
B. Nausea and vomiting
C. Liver function test abnormalities
D. Allergic reactions

**Core Concept:** Methotrexate is an immunosuppressive and anti-folate drug used to treat various malignancies and autoimmune disorders. It works by inhibiting the enzyme dihydrofolate reductase (DHFR), which leads to a deficiency of folic acid and interference with DNA synthesis.

**Why the Correct Answer is Right:** D. Allergic reactions are not an early adverse effect of methotrexate, as they usually manifest much later, after prolonged exposure to the drug. Early adverse effects usually occur within the first few days or weeks of treatment initiation and are dose-dependent.

**Why Each Wrong Option is Incorrect:**

A. Mouth ulcers: This is a common early adverse effect of methotrexate, as it interferes with DNA synthesis and impairs the healing process.
B. Nausea and vomiting: These are also early adverse effects, as methotrexate's anti-folate action affects gastrointestinal tract cells, leading to nausea, vomiting, and sometimes diarrhea.
C. Liver function test abnormalities: Liver toxicity is a late adverse effect of methotrexate, typically presenting after several weeks or months of treatment. Early adverse effects focus on gastrointestinal symptoms and oral mucositis.

**Clinical Pearl:** Monitoring for early adverse effects is crucial to adjust the methotrexate dose, ensuring optimal treatment efficacy and minimizing toxicity. Regular monitoring of blood counts, liver function tests, and renal function tests is essential during methotrexate therapy.

**Correct Answer:** D. Allergic reactions (Option D) are not mentioned as an early adverse effect in the question. Early adverse effects include mouth ulcers, nausea, vomiting, and liver function test abnormalities.