**Question:** PAS does not stain:

PAS stands for Periodic Acid-Schiff stain, which is a commonly used histochemical technique to stain carbohydrates in tissues. The stain helps in visualizing the distribution and type of carbohydrates, particularly glycogen and certain types of mucopolysaccharides.

**Core Concept:**
PAS stain works by reacting with aldehydes and reducing substances in carbohydrates, which are then visualized as a purple or red color. The stain selectively binds to the carbohydrates based on their chemical structure and function. In the context of histology, PAS stain helps to identify and differentiate between various types of carbohydrates in cells and tissues.

**Why the Correct Answer is Right:**
PAS stain is selective for carbohydrates containing aldehydes or reducing substances, and it does not bind to all types of carbohydrates. In the correct answer options, we can eliminate those that are not selectively stained by PAS:

A. Proteins: Proteins are not selectively stained by PAS because they do not contain aldehydes or reducing substances. Proteins are typically stained by other histochemical techniques like Masson's Trichrome or Sirius red stain.

B. Lipids: Lipids do not contain aldehydes or reducing substances, so they are not selectively stained by PAS. Furthermore, lipids are typically stained by oil red O or Sudan stain, which are specific for staining neutral lipids and acidic lipids, respectively.

C. Nucleic acids: Nucleic acids are also not selectively stained by PAS, as they do not contain aldehydes or reducing substances. Staining of nucleic acids is achieved through other histochemical techniques like Feulgen stain or DNA stain.

D. Water: Water is not a substance and does not contribute to the staining reaction. The staining reaction is dependent on the presence of carbohydrates with aldehydes or reducing substances.

**Why Each Wrong Answer is Incorrect:**

A. Proteins, B. Lipids, and C. Nucleic acids are incorrect because they do not contain the necessary chemical groups for PAS staining.

D. Water is incorrect as it is not a substance that participates in the staining reaction, as the staining process is dependent on the presence of carbohydrates with aldehydes or reducing substances.

**Clinical Pearl:**

Understanding the selective nature of PAS staining is crucial in histopathology for identifying specific cellular components and tissues. Ensure to use appropriate histochemical techniques for staining other cellular components like proteins, lipids, and nucleic acids to get accurate results in histopathology studies.