## **Core Concept**
Formalin is a widely used preservative in histopathology for fixing and preserving tissue specimens. It works by cross-linking proteins and preventing decay, thereby maintaining the tissue's structural integrity for microscopic examination. The typical concentration of formalin used for this purpose is crucial for effective preservation without causing excessive tissue hardening.

## **Why the Correct Answer is Right**
The correct answer, **10%**, refers to a 10% neutral buffered formalin (NBF) solution, which is the standard concentration used for preserving histopathological specimens. This solution is composed of 10% formaldehyde (the active ingredient in formalin) in a buffered saline solution. The buffering helps maintain a pH close to physiological, which is essential for preserving the morphology of tissues and preventing autolysis.

## **Why Each Wrong Option is Incorrect**
- **Option A: 5%** - While lower concentrations of formalin might be used for certain specific applications or in some fixative mixtures, 5% is not the standard concentration for general histopathological preservation.
- **Option B: 20%** - A 20% formalin solution is much too concentrated for routine histopathology. Such a high concentration could lead to over-fixation and excessive hardening of tissues, making them difficult to section and stain properly.
- **Option D: 30%** - This concentration is even more extreme than 20% and would be completely unsuitable for histopathological examination due to the severe hardening and distortion of tissue structures it would cause.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that **10% neutral buffered formalin (NBF)** is the gold standard for preserving tissues for histopathological examination. It's essential for accurate diagnosis, as improper fixation can lead to artifacts that may mimic or obscure pathological changes.

## **Correct Answer: C. 10%**