An asymptomatic 23-year-old man has a positive PPD (>15 mm), with no prior testing, negative chest X-ray, and negative sputum. What is the next step?

• A. Isoniazid for 9 months
• B. Isoniazid + pyridoxine for 9 months
• C. Rifampin for 6 months
• D. Ethambutol for 9 months

Answer: (B)

The key concept is treating latent tuberculosis infection to prevent future reactivation. With a positive tuberculin skin test and a normal chest X-ray plus no symptoms, the infection is latent, not active. The best approach is to give isoniazid for nine months to eradicate the dormant bacilli and reduce the lifetime risk of developing active TB.

Pyridoxine (vitamin B6) is added because isoniazid can cause vitamin B6 deficiency, leading to peripheral neuropathy. Providing pyridoxine prevents this adverse effect while allowing the full preventive benefit of isoniazid.

Rationale for other regimens: rifampin for a shorter duration can be used in some LTBI situations, but the classic and most protective approach in this scenario is isoniazid with pyridoxine for nine months. Ethambutol isn’t used for latent TB, and isoniazid without pyridoxine still risks neuropathy.