**Core Concept**
Distichiasis refers to the condition where an extra row of eyelashes grows from the meibomian glands, which are typically involved in tear secretion, not hair growth. This abnormal hair growth can cause eyelid irritation and discomfort.

**Why the Correct Answer is Right**
In distichiasis, the meibomian glands, located in the tarsal plate of the eyelid, undergo ectopic differentiation, leading to the formation of a second row of eyelashes. This aberrant hair growth can cause mechanical irritation to the cornea and conjunctiva, leading to symptoms such as redness, discharge, and pain. The meibomian glands play a crucial role in the production of meibum, an oily substance that helps to lubricate the eye.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not relevant to the definition of distichiasis.
**Option B:** Ectropion refers to a condition where the eyelid turns outward, away from the eye, which is not related to distichiasis.
**Option C:** Trichiasis is a condition where the eyelashes grow inward, toward the eye, causing irritation and discomfort. While it shares some similarities with distichiasis, it is a distinct condition.

**Clinical Pearl / High-Yield Fact**
Distichiasis can be a complication of previous eyelid surgery, particularly after blepharoplasty or other procedures that involve the tarsal plate. It's essential for ophthalmologists to be aware of this potential complication and take steps to prevent it.

**Correct Answer: D. Distichiasis is a condition where an extra row of eyelashes grows from the meibomian glands.**