**Question:** Ipsilateral deviation of tongue in 12th nerve palsy is due to -

A. Facial nerve (CN VII) involvement
B. Hypoglossal nerve (CN XII) involvement
C. Trigeminal nerve (CN V) involvement
D. Oculomotor nerve (CN III) involvement

**Core Concept:**
The 12th cranial nerve, also known as the hypoglossal nerve (CN XII), is responsible for the motor innervation of the muscles of the tongue. In a situation involving ipsilateral deviation of the tongue, we need to identify the affected cranial nerve.

**Why the Correct Answer is Right:**
In a case of ipsilateral deviation of the tongue, the correct answer is B, as it involves the hypoglossal nerve (CN XII). This nerve controls the muscles on the same side of the tongue, making it responsible for tongue movement.

**Why Each Wrong Option is Incorrect:**
A) Facial nerve (CN VII) involvement, which controls the muscles on the opposite side of the tongue and would cause contralateral deviation.
C) Trigeminal nerve (CN V) involvement, which controls the muscles on both sides of the tongue and would not result in an isolated deviation.
D) Oculomotor nerve (CN III) involvement, which controls the muscles responsible for upward gaze and constriction of the ipsilateral eye, not tongue movement.

**Why Ipsilateral Tongue Deviation:**
In a situation where the tongue deviates towards the affected side, it is usually due to the involvement of the contralateral (opposite side) cranial nerve. However, in this question, we are discussing ipsilateral (same side) tongue deviation. This can occur in cases of hypoglossal nerve (CN XII) involvement, as CN XII controls the muscles on the same side of the tongue, leading to the observed deviation.

**Clinical Pearl:**
The hypoglossal nerve is a bilateral cranial nerve, which means it innervates both sides of the tongue. The ipsilateral deviation of the tongue is usually a result of the involvement of the contralateral (opposite side) cranial nerve. In this case, the correct answer is B, as it relates to the hypoglossal nerve (CN XII) controlling the muscles on the same side of the tongue, leading to tongue deviation.

Understanding the anatomy and function of cranial nerves is vital in diagnosing neurological disorders accurately. In this scenario, hypoglossal nerve involvement would lead to an ipsilateral tongue deviation, while contralateral (opposite side) cranial nerve involvement would result in contralateral tongue deviation.