**Core Concept**
Tuberculosis (TB) diagnosis relies on a combination of diagnostic tools, including skin tests, sputum examination for acid-fast bacilli (AFB), and culture methods like Bactec, which detect *Mycobacterium tuberculosis* infection or disease.

**Why the Correct Answer is Right**
Mantoux test (tuberculin skin test) is widely used in children and high-risk groups to detect latent or active TB infection, especially when sputum is not available. Sputum AFB staining (e.g., Ziehl-Neelsen) provides rapid detection of acid-fast bacilli in clinical specimens, aiding in diagnosis of pulmonary TB. Bactec is a liquid culture system that grows *M. tuberculosis* over weeks, offering definitive identification and drug susceptibility testing. Together, these tests form a comprehensive diagnostic approach in suspected TB cases.

**Why Each Wrong Option is Incorrect**
Option A: While Mantoux is used in children, it is not specific for active TB and may yield false positives; it is not the sole or most reliable test, especially in immunocompromised individuals.
Option B: Sputum AFB is sensitive but has low specificity and is often negative in early or extrapulmonary TB; it is a screening tool, not a confirmatory test.
Option C: Bactec is a culture method and not a standalone investigation; it is used after initial screening but is not the first-line test.

**Clinical Pearl / High-Yield Fact**
In suspected TB, a stepwise approach is essential: start with clinical evaluation, then use Mantoux or IGRA in children or high-risk patients, followed by sputum AFB and culture (e.g., Bactec) for confirmation. Culture remains the gold standard for definitive diagnosis.

✓ Correct Answer: D. All of the above