## **Core Concept**
The question tests knowledge of histological stains, specifically those used for identifying lipids. Lipids are a diverse group of macromolecules that include fats, oils, sterols, and waxes. Various stains are used in histology to differentiate and visualize specific tissue components.

## **Why the Correct Answer is Right**
The correct answer, **Sudan Black B**, is indeed a stain used for lipids. It is a lysosome (fat-soluble dye) used to stain lipids, fats, and waxes. This makes option **D. Sudan Black B** an incorrect choice for a stain "not used" for lipid.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Oil Red O is a stain used for detecting lipids, making it incorrect as an answer to the question about a stain not used for lipids.
- **Option B:** Osmium Tetroxide is used to fix and stain lipids, particularly in electron microscopy, which reacts with unsaturated fatty acids, making it incorrect as well.
- **Option C:** PAS (Periodic Acid-Schiff) is used to detect polysaccharides such as glycogen, and glycoproteins but not primarily for lipids, making it the correct answer to the question.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that **PAS (Periodic Acid-Schiff)** staining is particularly useful in diagnosing conditions like glycogen storage diseases and certain fungal infections. It's also crucial for identifying basement membrane thickening in diabetic nephropathy.

## **Correct Answer:** . PAS.