**Core Concept**
AFB (Acid-Fast Bacillus) smear examination is a crucial diagnostic tool for tuberculosis and other mycobacterial infections. To determine the presence or absence of mycobacteria, a specific number of fields must be examined to ensure accurate results.

**Why the Correct Answer is Right**
The correct answer is 100 fields because the National Tuberculosis Institute (NTI) guidelines recommend examining at least 100 fields of the AFB smear to minimize the risk of missing a positive result. This is because mycobacteria may be present in low numbers or be distributed unevenly across the slide. Examining 100 fields increases the likelihood of detecting AFB, especially in cases where the bacillary load is low.

**Why Each Wrong Option is Incorrect**
**Option A:** 20 fields is insufficient for accurate detection of AFB, as it increases the risk of false-negative results.
**Option C:** 50 fields may not be enough to reliably detect AFB, particularly in cases with low bacillary loads.
**Option D:** 200 fields is excessive and not recommended by any standard guidelines, as it would be impractical and time-consuming.

**Clinical Pearl / High-Yield Fact**
When performing an AFB smear, it's essential to remember that the quality of the stain and the expertise of the microscopist can significantly impact the accuracy of the results. Therefore, it's crucial to use high-quality staining techniques and to have experienced microscopists performing the examination.

**✓ Correct Answer: B. 100**