**Core Concept:** Antitubercular drugs are medications used to treat tuberculosis (TB) infections. Pregnant women are a special population that requires careful consideration when administering these drugs due to potential risks to the developing fetus.

**Why the Correct Answer is Right:** Pyrazinamide (PZA) is a key drug in the initial phase of TB treatment, primarily used to shorten the duration of this phase. However, it is associated with a high risk of causing severe birth defects if administered during the first trimester of pregnancy. The risk is particularly high due to its active transport across the placental barrier, leading to high concentrations in the fetal circulation.

**Why Each Wrong Option is Incorrect:**

A. Isoniazid (INH) is a key drug in the initial phase of TB treatment, but it is generally considered safe during pregnancy. Its risk profile is lower than that of PZA, and it does not cause the same level of placental transport issues.

B. Rifampicin (RMP) is typically safe during pregnancy, although it is generally avoided in the first trimester due to potential teratogenicity (increased risk of birth defects). The risk is lower than with PZA.

C. Ethambutol (EMB) is generally considered safe during pregnancy, although it should be used with caution in the first trimester. Its risk profile is lower than that of PZA.

D. Streptomycin (SM) is a key drug in the initial phase of TB treatment, but it is generally avoided during pregnancy due to potential teratogenicity and hearing impairment. The risk is higher than with PZA.

**Clinical Pearl:** When treating TB in pregnant patients, it is crucial to carefully balance the potential benefits of treating active TB versus the risks associated with each drug. A personalized treatment plan, including appropriate counseling, is essential to optimize maternal and fetal outcomes.

**Correct Answer:** D. Streptomycin (SM) is contraindicated in pregnancy due to its potential for causing teratogenicity and hearing impairment. Streptomycin crosses the placenta and can lead to severe hearing loss in the developing fetus, potentially causing lifelong consequences.