**Question:** Compound used for fixation of protozoa found in stool is:

A. Glutaraldehyde
B. Mercaptoethanol
C. Calcofluor white
D. Methylene blue

**Correct Answer:** A. Glutaraldehyde

**Core Concept:**
The process of preparing stool samples for microscopic examination involves the use of fixatives to preserve the structure and morphology of the pathogens, particularly protozoa, present in the stool. Fixatives help to immobilize the organisms, making them suitable for staining and identification.

**Why the Correct Answer is Right:**
Glutaraldehyde is a widely used fixative in the field of microbiology, including parasitological examination of stool samples. It is a strong aldehyde that reacts with amino groups of proteins, forming irreversible cross-links and thus fixing the microorganisms in place. Glutaraldehyde also has a high affinity for nucleic acids, which helps in preserving the genetic material of the parasites.

**Why Each Wrong Option is Incorrect:**

A. Mercaptoethanol (Option B) is an alcohol that is used as a fixative in some histology protocols. However, it is not specifically designed for protozoal fixation in stool samples, and other fixatives like glutaraldehyde are more commonly employed for this purpose.

C. Calcofluor white (Option C) is a fluorescent dye used for staining fungal cells and hyphae. It is not suitable for protozoal fixation in stool samples, as it does not fix the organisms effectively, and other fixatives like glutaraldehyde provide better results.

D. Methylene blue (Option D) is a basic dye commonly used for staining cells and tissues. It is not a suitable fixative for protozoa in stool samples, as it does not preserve the structure and morphology of the organisms, and other fixatives like glutaraldehyde offer more reliable results for protozoal identification.

**Clinical Pearls:**
1. In parasitological examination of stool samples, the choice of fixative is crucial for accurate identification of parasites, particularly protozoa.
2. Glutaraldehyde-based fixatives provide better preservation of the organism's structure, enabling accurate identification and diagnosis of parasites in stool samples.