**Question:** Which of the following is a negative stain -

A. Haematoxylin
B. Eosin
C. Alcian blue
D. Toluidine blue

**Correct Answer:** D. Toluidine blue

**Core Concept:** Negative staining in histopathology refers to the technique where a counterstain is used to enhance the contrast between the sample and the background, making the structures of interest more visible. Negative stains are typically chosen based on their ability to bind to specific molecules in the sample and change their color, thus making the background appear darker.

**Why the Correct Answer is Right:** Toluidine blue is a widely used histological stain for negative staining. It binds to negatively charged molecules, such as DNA and RNA, in tissues. The stain's cationic nature allows it to bind to the negatively charged phosphate backbone of the nucleic acids, enhancing the contrast between the stained structures and the unstained background. This makes the background appear darker, making the desired structures more visible.

**Why Each Wrong Option is Incorrect:**

**A. Haematoxylin:** Haematoxylin is a positive stain, which binds to the basic amino acids present in the cell nuclei and cytoplasm, making them appear blue or purple. It is not a negative stain.

**B. Eosin:** Eosin is another positive stain, which binds to proteins and makes them appear pink or red. Negative staining is achieved by using a stain that binds to specific molecules in the sample, making the background appear darker. Eosin does not fulfill this criterion as it binds to proteins, not negatively charged molecules like Toluidine blue does.

**C. Alcian blue:** Alcian blue is a positive stain, which binds to mucopolysaccharides and glycoproteins in tissues, making them appear blue or purple. It is also not a negative stain.

**Clinical Pearls:** Negative staining is essential in histopathology to enhance the visibility of specific structures in tissue sections. Toluidine blue is a versatile stain used for various applications, including negative staining, particularly for evaluating the extracellular matrix and cell membranes. Negative staining aids in better visualization of tissue architecture and facilitates identification of cellular and extracellular structures in histopathology.