**Core Concept**
The earliest feature of tuberculosis (TB) refers to the initial manifestation of the disease, which can be subtle and often precedes the classical symptoms. In pulmonary TB, the earliest changes occur in the middle and lower lung zones due to gravity-dependent sedimentation of particles.

**Why the Correct Answer is Right**
The earliest feature of TB is often a non-specific cough, which may be dry or productive of blood-tinged sputum. This occurs due to the initial inflammatory response to the Mycobacterium tuberculosis infection, leading to the activation of immune cells and the release of cytokines. The cough is usually accompanied by subtle radiographic changes, such as fine reticulonodular infiltrates.

**Why Each Wrong Option is Incorrect**
**Option A:** While fever can be a symptom of TB, it is not the earliest feature, as it may occur later in the disease process.
**Option B:** Chest pain is a common symptom of TB, but it is not the earliest feature, which is often a non-specific cough.
**Option C:** Weight loss is a consequence of advanced TB, but it is not the earliest feature.

**Clinical Pearl / High-Yield Fact**
The "Rule of 3" in TB is a useful clinical pearl: 3 months of continuous cough, 3 liters of weight loss, and 3 areas of lung involvement are associated with a poor prognosis.

**Correct Answer: B. Chest pain.**