Okay, let's tackle this question about Methotrexate. The question is asking which agent should be given alongside Methotrexate to decrease its side effects. The correct answer is folic acid, right? Wait, but I remember that leucovorin is also used. Hmm, maybe I need to recall the exact mechanism.

Methotrexate is a folate antagonist, so it inhibits dihydrofolate reductase. This blocks the synthesis of tetrahydrofolate, which is needed for purine and thymidine synthesis. By inhibiting these, it's effective in treating cancer and autoimmune diseases. But the problem is the toxicity. So, to reduce toxicity, we give something that can bypass the blocked pathway. Folic acid itself is a precursor, but maybe it's not enough. Leucovorin, which is a reduced form of folic acid, can directly participate in the folate cycle without needing the enzyme that Methotrexate inhibits. That makes sense because leucovorin can provide the necessary tetrahydrofolate, thus protecting normal cells while allowing Methotrexate to target the cancer cells. So the correct answer should be leucovorin. Wait, but sometimes folic acid is given in lower doses for autoimmune conditions. Let me check that. In high-dose Methotrexate therapy, leucovorin is the rescue agent. In lower doses for conditions like rheumatoid arthritis, folic acid supplementation is used to reduce side effects like gastrointestinal upset. So depending on the context, both folic acid and leucovorin can be correct, but in the context of decreasing side effects, which is more appropriate? The question doesn't specify high-dose vs low-dose. But generally, when they ask about decreasing side effects of Methotrexate, the answer is folic acid or leucovorin. Since the options here are A to D but not provided, the correct answer in standard exams is often folic acid. Wait, but in high-dose MTX, leucovorin is used. So maybe the question is about standard doses, hence folic acid. Let me confirm. For example, in rheumatoid arthritis, folic acid is given to reduce toxicity. In oncology, leucovorin is used for high-dose MTX. Since the question is about decreasing side effects in general, the answer is folic acid. So the correct answer is folic acid. The other options might include B12, which isn't related, or something like thiamine. So the explanation should clarify that folic acid is used in lower doses, leucovorin in higher doses. The key is that both are forms of folate, but used in different contexts. So the answer here is folic acid.

**Core Concept** Methotrexate is a folate antagonist that inhibits dihydrofolate reductase, blocking tetrahydrofolate synthesis. This disrupts nucleotide synthesis, causing cytotoxicity. **Folic acid** or **leucovorin** (reduced folate) is used to rescue normal cells from toxicity while preserving antitumor effects.

**Why the Correct Answer is Right** Folic acid supplementation provides exogenous folate, bypassing the enzyme inhibition caused by methotrexate. In

✓ Correct Answer: D. Folinic acid