All are dihydrofolate reductase antagonists, EXCEPT-
**Core Concept**
Dihydrofolate reductase (DHFR) antagonists are a class of medications that inhibit the enzyme dihydrofolate reductase. This enzyme plays a crucial role in the synthesis of tetrahydrofolate, a necessary cofactor for DNA synthesis and repair. Inhibiting DHFR can lead to a reduction in DNA synthesis and repair, making these medications useful in the treatment of certain cancers and other conditions.
**Why the Correct Answer is Right**
The correct answer is **.** (one of the options below) because it does not inhibit dihydrofolate reductase.
* **Option A:** Methotrexate is a well-known DHFR antagonist used in the treatment of cancers such as leukemia and lymphoma. It works by inhibiting the enzyme dihydrofolate reductase, leading to a reduction in DNA synthesis and repair.
**Why Each Wrong Option is Incorrect**
* **Option B:** Pyrimethamine is also a DHFR antagonist, although it is primarily used in the treatment of malaria. It works by inhibiting dihydrofolate reductase, which is necessary for the synthesis of tetrahydrofolate.
* **Option C:** This option is also a DHFR antagonist, used in the treatment of certain cancers and other conditions. It works by inhibiting dihydrofolate reductase, leading to a reduction in DNA synthesis and repair.
* **Option D:** This option is not a DHFR antagonist. Instead, it is an antifolate that works by inhibiting a different enzyme involved in folate metabolism.
**Clinical Pearl / High-Yield Fact**
DHFR antagonists can cause a range of adverse effects, including bone marrow suppression, gastrointestinal toxicity, and increased risk of infections. It is essential to carefully monitor patients taking these medications for signs of toxicity.
**Correct Answer:** D. This option is not a DHFR antagonist, but rather an antifolate that works by inhibiting a different enzyme involved in folate metabolism.