**Core Concept**
The World Health Organization (WHO) defines a "case" in tuberculosis (TB) as a person with a positive sputum smear or culture result, indicating the presence of Mycobacterium tuberculosis. This definition is crucial for surveillance and control efforts.

**Why the Correct Answer is Right**
A person with a positive sputum smear or culture result is considered a "case" because it directly indicates the presence of M. tuberculosis in their respiratory secretions. Sputum smears are typically examined for acid-fast bacilli (AFB) using the Ziehl-Neelsen stain, while cultures are used to isolate and identify the bacteria. A positive result in either test confirms the diagnosis of active TB. This is in contrast to being Mantoux (PPD) positive, which indicates latent TB infection or previous exposure to the bacteria.

**Why Each Wrong Option is Incorrect**
**Option A:** Cough is a common symptom of TB, but it is not a definitive diagnostic criterion. Many conditions cause cough, and it is not specific to TB.
**Option B:** A Mantoux (PPD) test is used to diagnose latent TB infection or previous exposure to the bacteria. While it is an essential tool in TB control, a positive result does not confirm active TB disease.
**Option D:** An X-ray may show abnormalities suggestive of TB, such as upper lobe infiltrates or cavitations, but it is not a definitive diagnostic test. Many conditions can cause similar radiographic findings.

**Clinical Pearl / High-Yield Fact**
The WHO's definition of a "case" in TB is crucial for surveillance and control efforts, as it helps track the spread of the disease and evaluate the effectiveness of interventions.

**✓ Correct Answer: C. Sputum positive**