**Question:** Reactivation of TB affects

A. Lymph nodes
B. Respiratory system
C. Gastrointestinal tract
D. Skin

**Core Concept:** Tuberculosis (TB) is an infectious disease caused by Mycobacterium tuberculosis, which primarily affects the lungs. It can be latent (inactive) or active. Reactivation occurs when an individual with latent TB infection (LTBI) develops active TB disease due to a weakening of the immune system.

**Why the Correct Answer is Right:** Reactivation of TB primarily affects the respiratory system (B) since it is the primary site where Mycobacterium tuberculosis colonizes and causes disease. In the context of the question, reactivation occurs when the immune system fails to control the infection, leading to a breakdown in the immune response and a risk of active TB disease.

**Why Each Wrong Option is Incorrect:**

A. Lymph nodes (L) are involved in TB, particularly in disseminated TB. However, the correct answer (respiratory system) is more specific to the primary site of infection and disease progression.

C. Gastrointestinal tract (GI) involvement is rare in reactivated TB. While TB can affect other organs, including the GI tract, the question specifically asks about reactivation.

D. Skin (S) involvement in TB is uncommon and not directly related to reactivation. Although skin tuberculosis (TB skin test) is used to diagnose LTBI, reactivation refers to the progression of LTBI to active TB disease in the respiratory system.

**Clinical Pearl:** Understanding the correct answer helps medical students and practitioners recognize the clinical manifestations of TB and its impact on different organ systems. It emphasizes the importance of recognizing risk factors for LTBI reactivation, such as immune system compromise, and initiating appropriate preventive measures or treatments.