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Cranial nerve palsies matter because the exact pattern of eye or facial weakness often points to the underlying cause, not just the damaged nerve. Take two patients with facial weakness: one cannot wrinkle their forehead at all, the other can raise both eyebrows despite drooping on one side. That single sign, forehead sparing, decides whether the problem is a stroke or Bell's palsy before any scan is done. For example, imagine an oculomotor nerve (cranial nerve III) palsy: the eye sits 'down and out', the eyelid droops (ptosis), and a fixed dilated pupil raises urgent concern for a compressive cause such as an aneurysm (an abnormal bulge in a blood vessel wall).

Cranial nerve III (oculomotor) palsy causes a 'down and out' eye position and ptosis (drooping eyelid). If the pupil is also affected, it becomes fixed and dilated. A surgical cause, such as a posterior communicating artery aneurysm, compresses the pupil-carrying outer fibres first, so the pupil dilates early. A medical cause, such as diabetic microvascular ischaemia (reduced blood flow from small-vessel diabetic damage), damages the nerve's core first and typically spares the pupil. Cranial nerve IV (trochlear) palsy weakens the superior oblique eye muscle, causing vertical diplopia (double vision) that is worse looking down. Picture a patient who trips on stairs or struggles reading because the double vision worsens on downgaze. Cranial nerve VI (abducens) palsy weakens abduction (outward eye movement). Because this nerve has the longest intracranial course, it is a classic false-localising sign in raised intracranial pressure, meaning the palsy can appear far from the actual site of the problem. Cranial nerve VII (facial) palsy differs sharply by pattern. A lower motor neurone (LMN) lesion, as in Bell's palsy, weakens the whole half of the face including the forehead, because each forehead muscle receives cortical input from one side only. An upper motor neurone (UMN) lesion, as in stroke, spares the forehead, because it receives crossed input from both hemispheres. In practice, forehead sparing versus forehead involvement is the fastest bedside way to separate a stroke-related facial droop from Bell's palsy.

• A woman with lower motor neurone weakness of the whole left side of her face also reports loss of taste over the front of her tongue. Which nerve carries that taste?
• A woman has brief electric-shock pains over her cheek and upper jaw, triggered by chewing. Which division of the fifth cranial nerve is most likely involved?
• A man loses his sense of smell after falling and striking the back of his head. Shearing of which nerve fibres at the cribriform plate best explains this?
• When a torch is swung between a patient's eyes, the left pupil widens rather than narrows as the light reaches it. Which finding does this describe?

• CN III (oculomotor) palsy, down-and-out eye with ptosis, unlike CN IV's isolated vertical diplopia.
• Surgical (pupil-involving) CN III palsy, fixed dilated pupil from compression, whereas medical (pupil-sparing) CN III palsy from diabetes spares the pupil.
• CN VI (abducens) palsy, failed abduction, and can be a false-localising sign in raised intracranial pressure, unlike a true focal lesion.
• Bell's palsy (LMN CN VII), weakens the whole half of the face including the forehead, whereas a stroke (UMN CN VII) spares the forehead.

• That nerve carries touch sensation from the tongue, not taste.
• The fifth nerve's motor supply serves chewing muscles, not this sensory area.
• That nerve carries taste from the posterior third of the tongue.
• That nerve supplies tongue muscles and carries no taste fibres.
• That nerve carries taste only from the epiglottic region.

• Localise the clue to the relevant anatomical level, cell type, pathway or molecular process.
• Map that structure or process to its normal function.
• Predict the deficit or clinical sign produced when it is disrupted.
• Reject options that belong to a neighbouring structure, pathway or level of explanation.