**Core Concept**
The Revised National TB Control Program (RNTCP) emphasizes the importance of accurate and rapid diagnosis of pulmonary tuberculosis (TB) to initiate effective treatment and prevent transmission. The mainstay of diagnosis involves a combination of clinical evaluation and microbiological confirmation.

**Why the Correct Answer is Right**
The mainstay of diagnosis of pulmonary TB in the RNTCP is sputum microscopy, which involves examining sputum samples under a microscope for the presence of Mycobacterium tuberculosis. This method is rapid, cost-effective, and widely available, making it an ideal choice for resource-limited settings. Sputum microscopy is based on the principle of acid-fast bacilli (AFB) staining, where Ziehl-Neelsen stain is used to highlight the acid-fast properties of M. tuberculosis.

**Why Each Wrong Option is Incorrect**
**Option A:** Chest X-ray is a useful tool for detecting pulmonary TB, but it is not the mainstay of diagnosis. While X-ray findings can suggest TB, they may not be specific or sensitive enough to confirm the diagnosis.
**Option B:** Clinical evaluation involves assessing symptoms, medical history, and physical examination findings. However, a diagnosis of pulmonary TB cannot be made solely based on clinical evaluation, as it may not be specific or sensitive enough to rule out other conditions.
**Option C:** CT scan is a more sensitive imaging modality than chest X-ray, but it is not the mainstay of diagnosis. CT scan may be used to evaluate complications of TB or to monitor treatment response, but it is not a primary diagnostic tool.

**Clinical Pearl / High-Yield Fact**
Sputum microscopy is a rapid and cost-effective method for diagnosing pulmonary TB, but it has a relatively low sensitivity, especially in patients with paucibacillary disease. Therefore, a negative result does not rule out TB, and further diagnostic testing may be required.

**Correct Answer: C. Sputum microscopy.**