**Core Concept:** Antituberculosis drugs are a class of medications used to treat tuberculosis (TB), a bacterial infection caused by Mycobacterium tuberculosis. The commonly used drugs include Isoniazid, Rifampicin, Ethambutol, Pyrazinamide, and Streptomycin. Each drug targets specific aspects of M. tuberculosis growth, survival, and dissemination.

**Why the Correct Answer is Right:** The correct answer, Streptomycin, is not given in pregnancy due to its significant teratogenic risk, meaning it can cause birth defects when administered during pregnancy. Streptomycin is a bacteriostatic drug that inhibits protein synthesis in M. tuberculosis, but its use during pregnancy increases the risk of hearing loss in the fetus.

**Why Each Wrong Option is Incorrect:**

A. Isoniazid is not typically contraindicated in pregnancy but is used with caution, as it can cause liver toxicity and is a potential source of hepatitis.

B. Rifampicin is also used with caution in pregnancy because of its potential to cause hepatotoxicity (liver toxicity) and skin rash. However, its teratogenic risk is lower than Streptomycin.

C. Ethambutol is generally safe in pregnancy, causing fewer side effects compared to Streptomycin and Isoniazid. Its primary mechanism involves inhibiting mycolic acid synthesis, which is essential for M. tuberculosis cell wall synthesis.

**Clinical Pearl / High-Yield Fact:** In pregnant women with TB, the preferred first-line drugs are Rifampicin, Isoniazid, Pyrazinamide, and Ethambutol (RIPE regimen). However, a thorough risk-benefit analysis is essential before initiating any TB treatment in pregnant patients, considering the potential risks to the mother and fetus.

**Correct Answer:** Streptomycin **.**