**Core Concept**
The confirmatory test for tuberculosis (TB) is crucial for diagnosing active TB infection in patients. It involves detecting the presence of Mycobacterium tuberculosis (M. tuberculosis) through various methods. The confirmatory test is essential for differentiating between latent TB infection and active TB disease.

**Why the Correct Answer is Right**
The correct answer is an acid-fast bacilli (AFB) smear using Ziehl-Neelsen staining, which is a widely used confirmatory test for TB. This test involves staining sputum or other respiratory specimens with carbol fuchsin, followed by decolorization with acid-alcohol. The AFB stain allows for the visualization of M. tuberculosis under a microscope, confirming the presence of the bacteria. The AFB smear has a high specificity but variable sensitivity, requiring careful interpretation of the results.

**Why Each Wrong Option is Incorrect**
* **Option A:** Chest X-ray is not a confirmatory test for TB, as it can show non-specific findings such as pneumonic infiltrates or lymphadenopathy.
* **Option B:** Sputum culture is a sensitive method for detecting M. tuberculosis but is not a confirmatory test, as it may take 2-6 weeks to yield results.
* **Option D:** Interferon-gamma release assays (IGRAs) are used to diagnose latent TB infection, not active TB disease.

**Clinical Pearl / High-Yield Fact**
The AFB smear is a crucial diagnostic tool in TB diagnosis, but its sensitivity is limited, especially in patients with paucibacillary disease. Therefore, a negative AFB smear result does not rule out TB, and other diagnostic tests such as sputum culture or molecular tests should be considered.

**Correct Answer:** C.