**Core Concept**
TB, or tuberculosis, is a bacterial infection caused by Mycobacterium tuberculosis that primarily affects the lungs but can spread to other parts of the body. The disease progresses through various stages, with the first sign being a critical indicator of its onset.

**Why the Correct Answer is Right**
The first sign of TB is often a dry cough, which is a non-specific symptom that may be caused by various conditions. However, when associated with other symptoms like fever, night sweats, and weight loss, it raises suspicion for TB. The dry cough is thought to be due to the irritation of the cilia lining the bronchi by the TB bacilli. The cilia's attempt to clear the irritants leads to inflammation and coughing.

**Why Each Wrong Option is Incorrect**
**Option A:** Fever is a non-specific symptom that can be seen in various infections, including TB, but it is not the first sign.
**Option B:** Night sweats are a common symptom of TB, but they usually occur after the initial dry cough.
**Option C:** Weight loss is a significant symptom of TB, especially in advanced cases, but it is not the first sign.

**Clinical Pearl / High-Yield Fact**
TB is a classic example of a 'great imitator,' meaning it can mimic symptoms of other conditions, making diagnosis challenging. Remember that a dry cough in the right clinical context, especially with systemic symptoms like fever and night sweats, should raise suspicion for TB.

**Correct Answer:** C.