What ocular tissue does the A-scan spike labeled A and B represent?

The A-scan spike labeled B represents the lens because this part of the A-scan provides information about the internal reflective surfaces of the eye. In an A-scan ultrasonography of the eye, various structures produce distinct echoes that are displayed as spikes on the graph. The lens is characterized by a strong reflective interface between its anterior and posterior surfaces, which creates a prominent spike in the A-scan data.

Typically, the spike associated with the lens appears after the corneal spike—given the order of the structures from the front to the back of the eye—and reflects the differing acoustic impedances of the cornea and lens. This strong reflection allows clinicians to determine the position and dimensions of the lens, which is critical for various diagnostic purposes, including assessing cataracts or for planning cataract surgery.

In contrast, other structures such as the cornea, retina, and sclera also create reflective spikes on the A-scan, but they are distinguished by their specific positions and characteristics in the data. For instance, the cornea produces the first spike due to its frontmost location in the eye, while the retina is located posteriorly and has distinct reflective properties that differ from those of the lens.