## **Core Concept**
The head mirror used in ENT (Ear, Nose, and Throat) OPD (Outpatient Department) is a crucial tool for examining patients. It utilizes the principle of reflection to provide illumination and visualization of the patient's ear, nose, and throat. The focal length of the head mirror is essential for determining its utility in clinical settings.

## **Why the Correct Answer is Right**
The correct answer, **B. 25 cm**, is right because a head mirror with a focal length of 25 cm is commonly used in ENT examinations. This focal length allows for an appropriate distance between the light source (usually a lamp) and the area being examined, providing sufficient illumination. The head mirror collects light from a distant source and focuses it onto a small area, enabling a detailed examination.

## **Why Each Wrong Option is Incorrect**
- **Option A:** 10 cm is too short for a practical focal length in this context, as it would require the light source to be very close to the examination site, which is not feasible with standard OPD lighting setups.
- **Option C:** 50 cm might be too long for effective illumination and visualization in a typical ENT OPD setting, where space and maneuverability are limited.
- **Option D:** 15 cm is shorter than the commonly used focal length and might not provide the optimal illumination and viewing distance for most ENT examinations.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the head mirror's focal length of **25 cm** is a standard for ENT examinations because it balances the need for illumination and the practicalities of the examination space. This makes **25 cm** a high-yield fact for medical students and practitioners.

## **Correct Answer: B. 25 cm**