Fourth cranial nerve is?

Appeared in: KPSC Staff Nurse - 2016

Explanation

The fourth cranial nerve is the Trochlear nerve, commonly abbreviated as CN IV.
It is a purely motor nerve responsible for innervating the superior oblique muscle of the eye.
The action of the superior oblique muscle is to move the eyeball downward and inward (intorsion and depression).
The Trochlear nerve is unique as it is the only cranial nerve that emerges from the dorsal (posterior) aspect of the brainstem.

Option A: The Trigeminal nerve is the fifth cranial nerve (CN V), not the fourth. It is a mixed nerve with both sensory and motor functions.
Option B: The Abducens nerve is the sixth cranial nerve (CN VI), not the fourth. It innervates the lateral rectus muscle, which abducts the eye.
Option D: The Facial nerve is the seventh cranial nerve (CN VII), not the fourth. It is a mixed nerve responsible for facial expressions, taste, and glandular secretions.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Identification and function of cranial nerves as background academic context rather than a clinical decision trigger.
A nurse assesses cranial nerve function to detect neurological deficits. Damage to the Trochlear nerve (CN IV palsy) can cause vertical diplopia (double vision, with one image above the other), which worsens when looking down, such as when reading or walking down stairs.
Patients with CN IV palsy often adopt a characteristic compensatory head tilt to the side opposite the lesion to minimize the double vision.
What if? If a patient presents with an inability to move their eye outward (laterally), the nurse would suspect a lesion of the Abducens nerve (CN VI), not the Trochlear nerve.

How to Approach the Question

First, identify the core of the question, which is a direct recall of anatomical knowledge: identifying the fourth cranial nerve.
Access your memory of the 12 cranial nerves and their corresponding numbers and names.
Use a mnemonic if helpful, such as "Oh Oh Oh To Touch And Feel Very Good Velvet, Ah Heaven". The fourth word, "To", stands for Trochlear.
Evaluate each option: Trigeminal is V, Abducens is VI, Trochlear is IV, and Facial is VII.
Select the option that correctly matches the fourth position, which is Trochlear.

Concept Tested & Keywords

QBn6kLM-Aio6kF2mGzix-C

E6 Anatomy Applied Ashalatha 4e p. 426-428

E6 Anatomy Grays 43e Vol 1 Part 3 p. 34-36

Practise the full KPSC Staff Nurse - 2016

Attempt every question from this paper in a timed mock, then review the full solution for each one.