**Core Concept**
The World Health Organization (WHO) case definition for tuberculosis (TB) is based on the presence of Mycobacterium tuberculosis in a patient's sputum, which is detected using acid-fast bacillus (AFB) staining. This definition is crucial for diagnosing and managing TB, especially in areas with high prevalence.

**Why the Correct Answer is Right**
The correct answer, **C. Sputum AFB positive**, is based on the WHO case definition for TB. Sputum AFB positivity indicates the presence of M. tuberculosis in the patient's sputum, which is a key criterion for diagnosing pulmonary TB. The AFB staining technique is a rapid and cost-effective method for detecting TB bacilli in sputum samples. A positive result on AFB staining suggests that the patient is infectious and requires treatment.

**Why Each Wrong Option is Incorrect**
**Option A:** X-ray positive is not a definitive criterion for diagnosing TB. While chest X-rays can show abnormalities consistent with TB, such as upper lobe infiltrates or cavitation, a normal X-ray does not rule out TB. Many patients with TB may have a normal chest X-ray, especially in the early stages of the disease.

**Option B:** Culture positive is a more sensitive and specific test for diagnosing TB than AFB staining. However, it takes longer to obtain culture results, which can delay treatment initiation. Culture positivity is not the primary criterion for defining a TB case.

**Option D:** Tuberculosis positive is a vague term that does not specify the diagnostic method used. It is not a standard case definition for TB.

**Clinical Pearl / High-Yield Fact**
The WHO case definition for TB emphasizes the importance of sputum AFB positivity as a key criterion for diagnosing pulmonary TB. This definition helps ensure that patients with TB receive timely and appropriate treatment, reducing the risk of transmission to others.

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