**Core Concept**
Methotrexate is a folate antagonist used in the treatment of various conditions, including cancer, autoimmune diseases, and ectopic pregnancy. Its mechanism of action involves inhibiting dihydrofolate reductase (DHFR), an enzyme crucial for DNA synthesis and repair. This action makes methotrexate contraindicated in pregnancy due to the risk of teratogenicity and fetal growth restriction.

**Why the Correct Answer is Right**
Methotrexate is a potent inhibitor of cell division, which is essential for fetal development. When administered during pregnancy, it can cause birth defects, such as craniofacial abnormalities, limb defects, and cardiovascular malformations. Additionally, methotrexate can induce premature closure of the ductus arteriosus, leading to pulmonary hypertension in the fetus. The use of methotrexate in pregnancy is associated with a high risk of miscarriage, growth restriction, and stillbirth.

**Why Each Wrong Option is Incorrect**
**Option B:** Cyclosporine is an immunosuppressant used to prevent rejection in organ transplant recipients. While it is teratogenic, its use in pregnancy is not as strongly contraindicated as methotrexate. Cyclosporine can cause fetal growth restriction and prematurity but is not as likely to cause congenital malformations.

**Option C:** Chloroquine is an antimalarial drug that can be used in pregnancy, especially in areas where malaria is endemic. However, it is contraindicated in the first trimester due to the risk of retinal toxicity and potential teratogenic effects. Chloroquine is generally safer than methotrexate in pregnancy.

**Option D:** Pyrazinamide is an antibacterial agent used in the treatment of tuberculosis. It is generally considered safe in pregnancy, especially when used in combination with other antitubercular drugs. Pyrazinamide is not as strongly contraindicated in pregnancy as methotrexate.

**Clinical Pearl / High-Yield Fact**
When prescribing medications in pregnancy, it is essential to consider the potential risks and benefits to both the mother and the fetus. The American College of Obstetricians and Gynecologists (ACOG) provides guidelines for safe medication use in pregnancy, and healthcare providers should consult these resources when making treatment decisions.

✓ Correct Answer: A. Methotrexate.