## Core Concept
Cilia are complex, hair-like structures found on the surface of certain eukaryotic cells, including those in the respiratory, reproductive, and auditory systems. They play a crucial role in the movement of substances along the cell surface, such as mucus in the respiratory tract. The movement of cilia is coordinated and rhythmic, depending on the dynein arms and microtubules within their structure.

## Why the Correct Answer is Right
The correct answer, **D.**, states that cilia are non-motile in primary ciliary dyskinesia. Primary ciliary dyskinesia (PCD), also known as Kartagener syndrome, is a genetic disorder characterized by defective cilia. In PCD, the cilia are either non-motile or have an abnormal movement pattern due to ultrastructural defects, such as the absence of dynein arms. This leads to impaired clearance of mucus and other substances, resulting in respiratory and other systemic complications.

## Why Each Wrong Option is Incorrect
**Option A:** Incorrect because it inaccurately describes a characteristic of cilia that does not apply to the question context.

**Option B:** Incorrect as it does not accurately relate to the known functions or characteristics of cilia relevant to the question.

**Option C:** Incorrect because, like option A, it does not accurately describe a characteristic of cilia relevant to the question about primary ciliary dyskinesia.

## Clinical Pearl / High-Yield Fact
A key clinical pearl is that primary ciliary dyskinesia often presents with a triad of chronic sinusitis, bronchiectasis, and situs inversus (Kartagener syndrome). The diagnosis can be confirmed through tests that assess ciliary function and ultrastructure. This condition highlights the critical role of cilia in human health and disease.

## Correct Answer Line
**Correct Answer: D.**