A 45 years old male patient complaints of double vision when walking upstairs. His past history is significant for Type-ll diabetes, well controlled with glyburide. The nerve most probably involved is

Answer: (a) Abducent nerve.* Patient is having an injury to the abducent nerve (CN VI).* It innervates the lateral rectus muscle. Lateral rectus muscle moves the eyeball laterally (outwards). Therefore, a lesion of the abducent nerve results in internal strabismus (medial deviation) and diplopia (double vision).* The innervation of extraocular muscles can be remembered by a simple mnemonic, S04, LR6, 03. Superior oblique is innervated by 4th cranial nerve (trochlear), Lateral rectus by 6th cranial nerve (abducent), and others by 3rd cranial nerve (oculomotor).* The other muscles are superior, inferior and medial recti, and inferior oblique muscles.* The oculomotor nerve also supplies to levator palpabrae and carries parasympathetic fibers.* In abducent nerve lesion, the eye cannot look outwards due to paralysis of the lateral rectus.* The eye is turned inwards when trying to look straight ahead, due to the unopposed action of the medial, superior and inferior recti muscles supplied by the oculomotor nerve.