Citrovorum factor is given along with all the following drugs except
**Core Concept**
Citrovorum factor, also known as leucovorin, is a form of folinic acid used to counteract the toxic effects of certain chemotherapeutic agents, particularly those that inhibit dihydrofolate reductase (DHFR). This enzyme is crucial for the synthesis of tetrahydrofolate, an essential cofactor in DNA synthesis and repair.
**Why the Correct Answer is Right**
Leucovorin is administered to rescue patients from the adverse effects of drugs like methotrexate, which inhibits DHFR, thereby disrupting DNA synthesis and leading to cytotoxicity. By providing exogenous folinic acid, leucovorin helps restore the normal folate pool, enabling the continuation of DNA synthesis and cell division.
**Why Each Wrong Option is Incorrect**
**Option A:** Methotrexate - This is incorrect because methotrexate is actually the drug that requires leucovorin rescue to counteract its toxic effects.
**Option B:** Fluorouracil - This is incorrect because leucovorin is often used in conjunction with fluorouracil to enhance its antineoplastic effects, rather than to counteract its toxicity.
**Option C:** Cyclophosphamide - This is incorrect because cyclophosphamide is an alkylating agent, and leucovorin is not typically used to counteract its toxic effects.
**Clinical Pearl / High-Yield Fact**
Leucovorin rescue is a crucial strategy to mitigate the toxic effects of DHFR inhibitors like methotrexate, and it is essential to administer leucovorin promptly to prevent severe mucositis, myelosuppression, and other complications.
**Correct Answer:** A. Methotrexate