**Core Concept:** Tuberculosis (TB) is an infectious disease caused by the bacterium Mycobacterium tuberculosis, which primarily affects the lungs (pulmonary TB). Extrapulmonary TB refers to infections outside the lungs. Common sites for extrapulmonary TB include the lymph nodes, bones, joints, central nervous system, and genitourinary system.

**Why the Correct Answer is Right:** The commonest site for extrapulmonary TB is the lymph nodes, specifically the cervical lymph nodes. These nodes are the first line of defense and act as a filter, allowing the tubercle bacilli to spread to other parts of the body through the bloodstream and lymphatic system.

**Why Each Wrong Option is Incorrect:**
A. **Option A (Cervical)**: Although cervical lymph nodes are involved, they are not the only site for extrapulmonary TB. Other lymph nodes, bones, joints, central nervous system, and genitourinary system can also be affected.
B. **Option B (Bone/Joint)**: Although bone and joint TB are less common, they are still involved in extrapulmonary TB. The correct answer is not limited to just bone and joint involvement, as other sites are also involved.
C. **Option C (Central Nervous System)**: Central nervous system TB is a common site for extrapulmonary TB, but it is not the only option. Other sites are also involved.
D. **Option D (Genitourinary System)**: Although genitourinary TB is less common, it is still a site involved in extrapulmonary TB. The correct answer is not limited to just genitourinary system involvement, as other sites are also involved.

**Clinical Pearl:** TB can affect multiple organs, and a thorough clinical examination and diagnostic tests are crucial in determining the site of TB involvement.

**Correct Answer:** Extrapulmonary TB is common in the lymph nodes (Cervical). The correct answer focuses on the primary site of involvement, which is the lymph nodes, but acknowledges that other sites are also affected.