**Core Concept**
Lagophthalmos refers to the incomplete closure of the eyelids during blinking, leading to exposure of the conjunctiva and risk of dry eye or corneal damage. It is a clinical sign of eyelid malposition or dysfunction, often due to neurological or mechanical causes.

**Why the Correct Answer is Right**
Lagophthalmos specifically describes the failure of the upper and/or lower eyelids to close completely, resulting in persistent exposure of the eye surface. This is distinct from entropion (inward turning of the eyelid) or ectropion (outward turning), both of which involve structural deformity. Chalazion is a meibomian gland cyst, unrelated to eyelid closure. Incomplete palpebral closure is the defining feature of lagophthalmos, especially in conditions like facial palsy or neurological deficits.

**Why Each Wrong Option is Incorrect**
Option B: Chalazion is a chronic, painless, non-infectious meibomian gland cyst, not related to eyelid closure.
Option C: Entropion involves inward turning of the eyelid, causing lashes to rub the cornea, not failure of closure.
Option D: Ectropion involves outward turning of the eyelid, leading to exposure but not necessarily incomplete closure; it is a structural deformity, not a functional defect.

**Clinical Pearl / High-Yield Fact**
Lagophthalmos is a key sign in patients with facial nerve palsy or stroke, and it increases risk of corneal ulceration due to prolonged exposure. Always assess eyelid closure in patients with neurological disorders or after trauma.

✓ Correct Answer: A. Lagophthalmos