**Core Concept**
In uncomplicated pulmonary tuberculosis, close contacts of infected individuals are at high risk of developing active TB. Isoniazid (INH) prophylaxis is recommended for household contacts to prevent latent TB infection from progressing to active disease, especially in children.

**Why the Correct Answer is Right**
Children under 10 years, especially those with close contact to a TB patient, are at increased risk of developing latent TB infection. The World Health Organization (WHO) and CDC recommend isoniazid prophylaxis for household contacts, including siblings, to reduce transmission. A 3-year-old sibling is a high-risk contact and should receive INH prophylaxis to prevent latent TB from progressing to active disease. This is a standard preventive measure in pediatric TB management.

**Why Each Wrong Option is Incorrect**
Option B: Auditory function testing is not routinely required before starting anti-TB drugs. While ethambutol can cause optic neuritis, not auditory toxicity, and no evidence links it to hearing loss. This test is not a standard requirement in pediatric TB.
Option C: Polyarthritis is not a known adverse effect of isoniazid, rifampin, or ethambutol. While joint pain may occur, "polyarthritis" is not a recognized side effect of these drugs. The correct term is "polyarthritis" only in specific conditions like rheumatoid arthritis or drug-induced arthritis, not here.
Option D: The mother, as a close contact, is at risk and should receive INH prophylaxis. The statement that she does not need it is false. All household contacts, including caregivers, should be evaluated and given prophylaxis if appropriate.

**Clinical Pearl / High-Yield Fact**
All household contacts of a TB patient, especially children under 10 years, should receive isoniazid prophylaxis to prevent progression of latent TB to active disease. Prophylaxis should be initiated promptly after diagnosis in the index case.

✓ Correct Answer: A. His 3-year-old sibling should receive INH prophylaxis