**Core Concept**
Methotrexate is a folate antagonist that inhibits dihydrofolate reductase (DHFR), leading to impaired DNA synthesis and cell division. This property makes it contraindicated in pregnancy due to the risk of teratogenicity and fetal growth restriction.

**Why the Correct Answer is Right**
Methotrexate is classified as a teratogenic agent and is contraindicated in pregnancy. Its mechanism of action as a folate antagonist leads to impaired DNA synthesis, which is critical during fetal development. The use of methotrexate in pregnancy can result in congenital malformations, growth restriction, and even miscarriage. In contrast, the other options are used to manage SLE in pregnancy.

**Why Each Wrong Option is Incorrect**
**Option B:** Sulfasalazine is a disease-modifying antirheumatic drug (DMARD) that is used in the treatment of SLE, particularly for its anti-inflammatory and immunosuppressive properties. It is safe to use in pregnancy, especially in the third trimester.
**Option C:** Prednisolone is a corticosteroid that is commonly used to manage SLE due to its potent anti-inflammatory and immunosuppressive effects. It is often used in pregnancy, especially during the third trimester, to control disease activity.
**Option D:** Chloroquine is an antimalarial drug that has anti-inflammatory properties and is used in the treatment of SLE. It is considered safe to use in pregnancy, especially in the second and third trimesters, due to its low risk of teratogenicity.

**Clinical Pearl / High-Yield Fact**
It is essential to note that all medications should be used with caution in pregnancy, and their benefits should outweigh the risks. A thorough risk-benefit analysis should be performed before initiating any medication in a pregnant patient.

**✓ Correct Answer: A. Methotrexate. Methotrexate is contraindicated in pregnancy due to its teratogenic effects.**