## Core Concept
The hypoglossal nerve (cranial nerve XII) is responsible for controlling the intrinsic and extrinsic muscles of the tongue, except for the palatoglossus muscle. Unilateral (U/L) injury to the hypoglossal nerve affects the motor functions of the tongue on the same side.

## Why the Correct Answer is Right
A unilateral injury to the hypoglossal nerve leads to weakness of the ipsilateral tongue muscles. This results in difficulty protruding the tongue, with the tongue deviating towards the side of the lesion upon protrusion due to the action of the intact contralateral genioglossus muscle. However, it does not cause significant issues with swallowing (dysphagia) or speech articulation (dysarthria) initially, as these functions are primarily coordinated by other cranial nerves and muscle groups.

## Why Each Wrong Option is Incorrect
- **Option A:** Deviation of tongue to opposite side on protrusion - This is incorrect because with a unilateral hypoglossal nerve injury, the tongue deviates towards the side of the lesion upon protrusion, not to the opposite side.
- **Option B:** Difficulty in protruding the tongue - This is a correct effect of hypoglossal nerve injury; however, we are looking for "all except."
- **Option C:** Weakness of the ipsilateral tongue muscles - This is a direct consequence of hypoglossal nerve injury.
- **Option D:** Significant dysphagia - While mild dysphagia can occur, significant dysphagia is not typically a primary symptom of isolated hypoglossal nerve injury, as the primary muscles involved in swallowing are controlled by other cranial nerves.

## Clinical Pearl / High-Yield Fact
A key clinical feature to remember is that a unilateral hypoglossal nerve injury causes the tongue to deviate **towards** the side of the lesion when protruded, due to the unopposed action of the genioglossus muscle on the contralateral side.

## Correct Answer: D. Significant dysphagia.