With an A-mode display, the internal contents of a simple cyst will appear as:

• A. An area with very dark shades of gray
• B. An area with rapidly moving spikes
• C. An area with tall spikes surrounded on each end with areas absent of spikes
• D. An area with no spikes

Answer: (D)

In A-mode ultrasound, echoes appear as vertical spikes along a depth axis, with the spike height reflecting the echo strength. A simple cyst is fluid-filled and anechoic, meaning it produces essentially no internal reflections. So, inside the cyst there are no echoes to generate spikes; only the cyst walls create spikes at their interfaces. Therefore the interior looks like a flat baseline with no spikes, while the walls show spikes. The other descriptions imply internal echoes, motion, or grayscale imaging—features not present in a simple cyst on A-mode.