**Core Concept**
In extra-pulmonary tuberculosis (TB), the disease spreads from the primary site in the lungs to other parts of the body through the bloodstream or lymphatic system. This can involve various organs and tissues, including the lymph nodes, gastrointestinal tract, skeletal system, and genitourinary tract. The clinical presentation and diagnostic tests help identify the affected site.

**Why the Correct Answer is Right**
The lymph nodes are the most common site affected in extra-pulmonary TB, particularly in cases where the disease spreads through the lymphatic system. This is because the lymph nodes act as filters for the lymph fluid, and TB bacteria can easily colonize these areas. In this patient, the presence of night sweats and significant weight loss, along with the negative BAL (Bronchoalveolar Lavage) result, suggests that the disease has spread beyond the lungs. The lymph nodes are the most likely site of involvement due to their proximity to the primary site of infection and their role in filtering the lymph fluid.

**Why Each Wrong Option is Incorrect**
**Option A:** While the skeletal system can be affected in TB, it is less common than lymph node involvement in extra-pulmonary TB. Skeletal TB typically presents with bone pain, joint pain, or deformity, which are not mentioned in the patient's symptoms.
**Option B:** Gastrointestinal TB can occur, but it is relatively rare and often presents with symptoms such as abdominal pain, diarrhea, or weight loss. The patient's symptoms are more suggestive of lymph node involvement.
**Option D:** Genitourinary TB can also occur, but it typically presents with symptoms such as urinary tract infections, kidney stones, or infertility. The patient's symptoms do not strongly suggest genitourinary involvement.

**Clinical Pearl / High-Yield Fact**
When suspecting extra-pulmonary TB, it's essential to consider the patient's clinical presentation, including symptoms such as night sweats, weight loss, and fever. The negative BAL result in this case suggests that the disease has spread beyond the lungs, making lymph node involvement a more likely diagnosis.

**✓ Correct Answer: C. Lymph nodes**