Okay, let's tackle this question. The user is asking about the WHO protocol for antitubercular treatment (ATT) drugs that are contraindicated during pregnancy. First, I need to recall the WHO guidelines on this topic.

The core concept here is drug safety during pregnancy. Certain medications can be teratogenic, meaning they can cause harm to the developing fetus. The WHO classifies drugs into categories based on their safety during pregnancy. For tuberculosis treatment, some drugs are considered safe, while others are contraindicated.

The correct answer would be the drug that's contraindicated. Let's think about the common ATT drugs. First-line drugs include isoniazid, rifampin, pyrazinamide, ethambutol, and streptomycin. Among these, streptomycin is an aminoglycoside. Aminoglycosides are known to cause ototoxicity in the fetus, especially during the first trimester. The WHO typically lists streptomycin as contraindicated in pregnancy. So if one of the options is streptomycin (C), that's likely the answer.

Now, the wrong options: Isoniazid and rifampin are generally considered safe in pregnancy. Pyrazinamide is also used in pregnancy, though there's some debate, but WHO usually includes it in the safe category. Ethambutol is also considered safe, though there's a risk of optic neuritis, but that's more of a side effect than a contraindication in pregnancy.

So the clinical pearl is to remember that aminoglycosides like streptomycin are contraindicated in pregnancy due to fetal ototoxicity. The correct answer is C.

**Core Concept**
The World Health Organization (WHO) classifies antitubercular drugs (ATT) based on safety during pregnancy. Drugs contraindicated in pregnancy (Category C/L) include those with proven fetal toxicity, particularly **aminoglycosides like streptomycin**, which cause **8th cranial nerve damage** in the fetus.

**Why the Correct Answer is Right**
Streptomycin, a first-line ATT drug, is contraindicated in pregnancy due to its ototoxic effect on the developing fetus. It crosses the placenta and can lead to **sensorineural hearing loss** in the fetus, especially during the first trimester when otic development is active. The WHO explicitly lists it as Category C/L (contraindicated in pregnancy).

**Why Each Wrong Option is Incorrect**
**Option A:** Isoniazid is safe in pregnancy (Category A) and remains a cornerstone of ATT.
**Option B:** Rifampin is also safe (Category A) and is used in pregnant TB patients.
**Option D:** Pyrazinamide is WHO-recommended for use in pregnancy (Category A), despite limited data, due to its efficacy in TB treatment.

**Clinical Pearl / High-Yield Fact**
**"Streptomycin = Silent S = Silent harm to the fetus"** — Remember that aminoglycosides cause fetal ototoxicity. Always avoid streptomycin in pregnancy; use ethambutol or pyrazinamide instead if needed.

**Correct Answer: C. Streptomycin**

✓ Correct Answer: D. Streptomycin