**Core Concept**
Distichiasis is a rare congenital or acquired condition characterized by the presence of an extra row of eyelashes. This condition can cause irritation, foreign body sensation, or even corneal damage due to the abnormal eyelashes. The condition is often associated with genetic syndromes or other ocular abnormalities.

**Why the Correct Answer is Right**
The correct answer, **B. Abnormal extra row of cilia**, is the defining characteristic of distichiasis. This condition occurs when an extra row of eyelashes grows from the meibomian glands or the tarsal plate, leading to an abnormal arrangement of the eyelashes. The extra row of cilia can cause discomfort and vision problems if left untreated.

**Why Each Wrong Option is Incorrect**
**Option A:** Abnormal inversion of eyelashes refers to entropion, a condition where the eyelid rolls inward, causing the eyelashes to rub against the cornea. This is a different condition from distichiasis.
**Option C:** Abnormal eversion of eyelashes refers to ectropion, a condition where the eyelid rolls outward, exposing the sclera and causing discomfort. Again, this is a different condition from distichiasis.
**Option D:** Misdirected cilia is a vague term that could refer to any number of conditions, including entropion or distichiasis. However, it does not specifically describe the abnormal extra row of cilia characteristic of distichiasis.

**Clinical Pearl / High-Yield Fact**
Distichiasis can be associated with other ocular conditions, such as blepharospasm, lagophthalmos, or ptosis. It is essential to diagnose and treat distichiasis promptly to prevent complications like corneal ulcers or vision loss.

**✓ Correct Answer: B. Abnormal extra row of cilia**