Under passive surveillance for tuberculosis, indication for sputum microscopy is cough for______ weeks or more.
**Core Concept**
Passive surveillance for tuberculosis involves monitoring patients who are at risk of developing active TB or who are in close contact with confirmed TB cases. Sputum microscopy is a diagnostic tool used to detect Mycobacterium tuberculosis in respiratory secretions. The indication for sputum microscopy in this context is a cough lasting for a specified duration, which serves as a surrogate marker for potential TB infection.
**Why the Correct Answer is Right**
The correct answer is based on the World Health Organization's (WHO) guidelines for TB control, which recommend sputum microscopy for patients with a cough lasting 2 weeks or more. This duration is chosen because it is long enough to be indicative of pulmonary TB, but not so long that it would include patients with acute respiratory infections or other conditions that may also cause cough. The mechanism underlying this recommendation is based on the fact that pulmonary TB typically causes a persistent cough, which is a key symptom used to identify patients who require further evaluation.
**Why Each Wrong Option is Incorrect**
**Option A:** A cough lasting less than 2 weeks is not sufficient to warrant sputum microscopy, as it may be due to a self-limiting respiratory infection or other conditions that do not require TB diagnosis.
**Option B:** A cough lasting 3 weeks or more is indeed an indication for sputum microscopy, but it is not the answer choice given.
**Option D:** This option is not relevant to the question, as it does not provide a specific duration of cough.
**Clinical Pearl / High-Yield Fact**
In passive surveillance for TB, a cough lasting 2 weeks or more is a key indicator for sputum microscopy, and healthcare providers should consider this symptom when evaluating patients for potential TB infection.
**Correct Answer: A. 2 weeks.**