**Core Concept**
Distichiasis refers to a rare congenital condition characterized by the presence of an extra row of eyelashes, which can cause irritation and discomfort to the eye. This condition arises due to the abnormal development of the eyelid margin during embryogenesis.

**Why the Correct Answer is Right**
In distichiasis, the extra row of eyelashes grows from the meibomian gland orifices, which are normally responsible for the secretion of meibum. The abnormal growth of these eyelashes can lead to mechanical irritation of the cornea and conjunctiva, causing symptoms such as photophobia, tearing, and discomfort. This condition can be managed with surgical removal of the extra lashes.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately describe distichiasis. Trichiasis, on the other hand, refers to the inward turning of eyelashes, which can also cause discomfort and irritation.

**Option B:** This option is incorrect because ectropion refers to the outward turning of the eyelid margin, which can expose the conjunctiva and cause symptoms such as tearing and discomfort.

**Option C:** This option is incorrect because entropion refers to the inward turning of the eyelid margin, which can also cause irritation and discomfort due to the rubbing of the eyelashes against the cornea.

**Clinical Pearl / High-Yield Fact**
It is essential to differentiate between distichiasis, trichiasis, ectropion, and entropion to provide appropriate management and treatment for patients presenting with eyelid abnormalities.

**Correct Answer: D. Distichiasis refers to the presence of an extra row of eyelashes.**