A 45-year-old man presents with left-sided mild ptosis (1–2 mm), miosis, and anhidrosis of the left face and neck. Imaging reveals a mass at the left lung apex compressing the sympathetic chain. The structure marked **C** in the diagram is responsible for the ptosis in this patient. Which of the following best explains the mechanism of ptosis in this condition?

Explanation

## Why "Loss of sympathetic innervation to Müller muscle..." is right The structure marked **C** (Müller muscle) is a smooth muscle of the superior tarsal plate innervated exclusively by the sympathetic nervous system. It contributes approximately 2 mm of upper eyelid elevation. In Horner syndrome caused by preganglionic sympathetic lesion (2nd-order neuron damage, as in Pancoast tumor compressing the sympathetic chain at T1), loss of sympathetic tone to Müller muscle results in mild ptosis of 1–2 mm—a hallmark of Horner syndrome. The clinical triad of Horner (ptosis + miosis + anhidrosis of face and neck in 2nd-order lesion) matches this patient's presentation exactly. This is distinct from CN III palsy, which affects the main levator palpebrae superioris and causes severe, often complete ptosis (3–4 mm) with "down and out" eye and pupillary dilation (Gray's Anatomy 42e Ch 41; AK Khurana Ophthalmology 7e). ## Why each distractor is wrong - **"Compression of the oculomotor nerve (CN III)..."**: CN III palsy causes severe ptosis (3–4 mm), often complete, with "down and out" eye position and dilated pupil—not the mild 1–2 mm ptosis seen here. The patient's normal pupil size rules out CN III involvement. - **"Denervation of the levator palpebrae superioris..."**: The levator palpebrae superioris is innervated by CN III (parasympathetic), not the sympathetic system. Its denervation would cause severe ptosis and eye depression, not the mild ptosis of Horner syndrome. - **"Paralysis of the orbicularis oculi..."**: The orbicularis oculi (marked **A**) is innervated by CN VII and is responsible for eyelid closure, not elevation. Its paralysis causes inability to close the eye, not ptosis. **High-Yield:** Horner syndrome ptosis is mild (1–2 mm) due to loss of sympathetic-innervated Müller muscle; CN III ptosis is severe (3–4 mm) due to levator paralysis. Anhidrosis pattern (face + neck in 2nd-order) localizes the lesion to preganglionic sympathetic chain. [cite: Gray's Anatomy 42e Ch 41; AK Khurana Ophthalmology 7e]