**Core Concept**
Metachromatic granules are intracellular, acidic storage particles found in certain bacteria, especially *Corynebacterium* species. These granules exhibit a color change when stained due to their ability to absorb and retain dye molecules, a property known as metachromasia.

**Why the Correct Answer is Right**
Ponder's stain is a specialized differential stain used to detect metachromatic granules in bacteria. It employs a mixture of methylene blue and other dyes that are absorbed by the acidic granules, causing them to appear blue or purple under the microscope. The granules take up the stain due to their high negative charge, leading to a visible metachromatic shift. This makes Ponder's stain uniquely effective for identifying such structures in clinical specimens.

**Why Each Wrong Option is Incorrect**
Option B: Negative stain does not involve dye uptake by granules and is used to visualize cell morphology, not internal structures like metachromatic granules.
Option C: Gram's stain differentiates bacteria based on cell wall structure and does not detect metachromatic granules.
Option D: Leishman stain is used for parasitic infections like leishmaniasis and does not produce metachromatic staining in bacteria.

**Clinical Pearl / High-Yield Fact**
Metachromatic granules are a key diagnostic feature in *Corynebacterium* infections, especially in chronic respiratory or skin infections. Always consider Ponder's stain when evaluating suspected bacterial granulomas or intracellular storage material.

✓ Correct Answer: A. Ponder's stain