A patient presents with sudden onset painless diplopia on downward gaze — especially when descending stairs — with difficulty reading. The affected muscle is:

• A Superior oblique (CN IV palsy) ✓
• B Inferior rectus (CN III palsy)
• C Lateral rectus (CN VI palsy)
• D Inferior oblique (CN III partial palsy)

Explanation

The superior oblique muscle (innervated by CN IV) primarily depresses the adducted eye (intorsion and depression). Its palsy is most symptomatic during downward and inward gaze (reading, descending stairs) because the superior oblique is the primary depressor when the eye is in the adducted position. Patients often adopt a compensatory head tilt away from the affected side. CN IV palsy (trochlear nerve) is the most common cause of vertical diplopia and the most common isolated cranial nerve palsy following closed head injury, as the trochlear nerve has the longest intracranial course and exits dorsally from the brainstem.

Reference: BD Chaurasia's Human Anatomy, 8th ed.

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Written and medically reviewed by the StethoPrep medical team.