**Question:** Hair on end appearance is seen in X-ray skull in

A. Frontal bossing
B. Occipital prominence
C. Zygomatic arch prominence
D. Nasal bone widening

**Core Concept:** Hair on end appearance refers to the characteristic X-ray appearance of a long bone, typically seen in children or adolescents with certain skeletal dysplasias or disorders. This appearance results from the irregular growth patterns and delayed ossification of the epiphyseal plates due to the underlying pathology.

**Why the Correct Answer is Right:** The correct answer is D. Nasal bone widening because this sign is associated with delayed closure of the nasal sutures, which leads to irregular growth and widening of the nasal bones. This occurs in conditions like Noonan syndrome, Down syndrome, and Turner syndrome, among others.

**Why Each Wrong Option is Incorrect:**

A. Frontal bossing is a normal developmental finding in children and does not present with the hair on end appearance.
B. Occipital prominence refers to the prominence at the back of the skull and is unrelated to the hair on end appearance.
C. Zygomatic arch prominence represents the prominent cheekbones and is unrelated to the hair on end appearance.

**Clinical Pearl:** The hair on end appearance is a crucial sign to recognize in clinical practice as it suggests a specific underlying diagnosis, allowing for targeted further investigations and management. This can significantly impact patient care and prognosis.