Cranial Nerve Lesions: MRCEM Primary questions with answers

Cranial Nerve Lesions asks what each of the twelve nerves supplies and what is lost when it is damaged. The topic runs from the olfactory and optic nerves through to the vagus, accessory and hypoglossal nerves. Anatomy carries 60 of the 180 questions in MRCEM Primary. In the emergency department this knowledge helps you explain a facial droop, double vision or an abnormal pupil after head injury or stroke.

5 free questions below, each with the answer and the full explanation · 112 in the full bank · Last updated after the 27 October 2026 MRCEM Primary sitting.

1. Question 1 · Cranial Nerve Vii: Facial Nerve

   A patient aged 45 presents with sudden onset of facial paralysis affecting one side. After examination, the diagnosis of Bell’s palsy is made. The most common cause of Bell’s palsy is:

   1. AAutoimmune
   2. BHereditary
   3. CTrauma
   4. DViral
   5. EIdiopathic
   Show answer and explanation

   Answer: E. Most cases of Bell's palsy have no definite cause and are classed as idiopathic, although viral reactivation is suspected.

   • A: Autoimmune mechanisms are not established as the usual cause.
   • B: Bell's palsy is not usually hereditary.
   • C: Trauma causes facial palsy but is not the commonest cause.
   • D: Viral infection is suspected, yet most cases are labelled idiopathic.

   Full explanation

   Bell’s palsy occurs due to inflammation and swelling of the seventh cranial nerve, which is responsible for controlling the muscles of the face. While specific conditions, such as viral infections, can lead to this inflammation, many instances of Bell’s palsy arise without a definitive cause, hence they are classified as idiopathic.

    

2. Question 2 · Cranial Nerve Ii: Optic Nerve

   A diabetic patient aged 30 presented to the emergency department with a two-day history of pain in her right eye worsened by eye movement. The diagnosis of optic neuritis was made by an ophthalmologist. What will be the cause of optic neuritis?

   1. ADemyelination
   2. BRetinal detachment
   3. CRetinitis
   4. DNeuromyelitis
   5. EHyperglycemia
   Show answer and explanation

   Answer: A. Optic neuritis is an acute inflammatory demyelinating disorder of the optic nerve, impairing saltatory conduction.

   • B: Retinal detachment is painless with flashes, floaters and field loss.
   • C: Retinitis is retinal inflammation, not nerve demyelination, and is not painful on movement.
   • D: Neuromyelitis optica is a rarer, specific condition; demyelination is the general cause.
   • E: Hyperglycaemia causes diabetic retinopathy, not painful optic neuritis.

   Full explanation: Cranial Nerve Ii: Optic Nerve

   Optic nerve:

   Transmit special sensory information from the retina of the eye to the primary visual cortex of the brain.  

   • Enters skull via optic canal of the sphenoid bone 

    

    

   Note:Relative afferent pupillary defect (RAPD): Seen in optic nerve disease or asymmetrical disease of the retina in which pupils respond differently to light stimuli shone in one eye at a time   

   • swinging light test if there is no RAPD the pupils of both eyes constrict equally regardless of which eye is stimulated by the light.  
   • In an abnormal swinging-light test if there is a RAPD there is less pupil constriction in the eye with the retinal or optic nerve disease  

   Clinical pearls:

   Pituitary adenoma: Pituitary gland lies in close proximity to optic chiasm,tumor of pituitary gland compresses optic nerve at chiasm produces visual defect called bitemporal hemianopia

    

3. Question 3 · Cranial Nerve V: Trigeminal Nerve

   A rugby player aged 20 attends ED with damage to the mastoid. On further examination you find the jaw reflex is lost. Which of the following nerves is injured:

   1. AHypoglossal nerve
   2. BGlossopharyngeal nerve
   3. CVagus nerve
   4. DFacial nerve
   5. ETrigeminal nerve
   Show answer and explanation

   Answer: E. The trigeminal nerve is the afferent and efferent limb of the jaw jerk reflex, tested clinically.

   • A: The hypoglossal nerve supplies tongue movement.
   • B: The glossopharyngeal nerve is the afferent limb of the gag reflex.
   • C: The vagus nerve is the efferent limb of the gag reflex.
   • D: The facial nerve supplies facial expression and is tested by the blink motor limb.

   Full explanation: Cranial Nerve V: Trigeminal Nerve

   Trigeminal nerve:

   Mixed motor and sensory nerve. It is also the largest cranial nerve. Arises from several nuclei in the brainstem, exits brainstem from pons It has three main divisions: 

   1: Opthalmic V1 

   2: Maxillary V2 

   3: Mandibular V3 

4. Question 4 · Cranial Nerve Xi: Spinal Accessory Nerve

   The mandibular division of the trigeminal nerve leaves the skull through the:

   1. AForamen ovale
   2. BInternal acoustic meatus
   3. CJugular foramen
   4. DForamen rotundum
   5. EHypoglossal canal
   Show answer and explanation

   Answer: A. The mandibular nerve (V3) exits via the foramen ovale.

   • B: The internal acoustic meatus transmits CN VII and VIII.
   • C: The jugular foramen transmits CN IX, X and XI.
   • D: Foramen rotundum transmits the maxillary nerve (V2).
   • E: The hypoglossal canal transmits CN XII.

   Full explanation: Cranial Nerve Xi: Spinal Accessory Nerve

   Spinal Accessory Nerve (CN XI):

   Arises from cervical segments C1 – C5/C6, enters cranial cavity through foramen magnum, travels through posterior cranial fossa and exits skull through jugular foramen. 

   Outside the cranium, it descends along the internal carotid artery and innervate sternocleidomastoid muscle. 

   Cranial Nerve	Spinal Accessory Nerve (CN XI)
   Causes of injury	Iatrogenic (Most commonly), Procedures such as cervical lymph node excision biopsy or central line insertion can cause trauma to the nerve.
   Function	Motor: sternocleidomastoid, trapezius
   Assessment	Test head rotation and shoulder shrug against resistance
   Clinical effects of injury	inability to rotate head towards opposite side to the lesion (Weakened sternocleidomastoid Inability to shrug ipsilateral shoulder (weakened trapezium) and resulting shoulder droop

5. Question 5 · Cranial Nerve 1- Olfactory Nerve

   Concerning the olfactory nerve, which of these statements is CORRECT:

   1. AThe olfactory bulb lies in the posterior cranial fossa.
   2. BThe olfactory nerve is the longest cranial nerve.
   3. CThe olfactory nerve originates from the medulla oblongata.
   4. DThe olfactory nerve carries crude touch sensation from the nasal cavity.
   5. EThe olfactory tract runs inferiorly to the frontal lobe.
   Show answer and explanation

   Answer: E. The olfactory tract runs inferior to the frontal lobe.

   • A: The bulb lies in the anterior cranial fossa.
   • B: It is the shortest cranial nerve.
   • C: It originates from the cerebrum, not the medulla.
   • D: It carries smell, not touch.

   Full explanation: Cranial Nerve 1- Olfactory Nerve

   Olfactory nerve: 

   First and shortest cranial nerve transmits sensory information from the nasal epithelium via the cribriform plate of the ethmoid bone to the primary olfactory cortex 

   Cranial Nerve	Olfactory Nerve (CN I)
   Cause of injury	trauma most commonly (fractured ethmoid bone or head injury), Nasal disease, meningitis
   Function	Sensory: sense of smell
   Assessment	Enquire about changes in smell/taste
   Clinical effects of injury	Anosmia (loss of smell) and taste.