Which letter on the diagram indicates motile structural elements of the epithelial membrane?

• A. B: Microvilli
• B. C: Basement membrane
• C. D: Tight junctions
• D. A: Cilia

Answer: (D)

Verified step by step guidance

1. Understand the question: The problem is asking which structural element of the epithelial membrane is motile (capable of movement). The options provided are cilia, microvilli, basement membrane, and tight junctions.
2. Review the characteristics of each option: Cilia are hair-like structures that extend from the surface of certain epithelial cells and are motile, meaning they can move to propel substances over the epithelial surface. Microvilli are non-motile projections that increase surface area for absorption. The basement membrane is a non-cellular structure that provides support and anchorage for epithelial cells. Tight junctions are cell-to-cell connections that prevent the passage of substances between cells.
3. Focus on the key term 'motile': Since the question specifies motility, eliminate options that are not capable of movement. Microvilli, the basement membrane, and tight junctions are all non-motile structures.
4. Identify the correct answer: Based on the information, cilia are the only motile structural elements listed in the options. They are responsible for moving mucus, debris, or other substances across the epithelial surface in areas like the respiratory tract.
5. Confirm the answer: The correct answer is A: Cilia, as they are the only motile structures among the options provided.