With an aliased signal, the peak of the waveform is clipped off and appears on the opposite side of the baseline, indicating retrograde flow.

• A. Aliased signal and Retrograde flow
• B. Mirror image
• C. Bidirectional flow
• D. Spectral broadening

Answer: (A)

Doppler signal aliasing happens when the velocity being measured exceeds half the PRF, so the spectrum wraps around to the opposite side of the baseline. The peak getting clipped at that limit and reappearing on the other side is the telltale sign that the display is interpreting a fast forward velocity as backwards flow. That combination—an aliased signal with flow appearing retrograde—is exactly what aliasing produces. Other options describe different display phenomena (a mirror image, bidirectional real flow, or spectral broadening) and don’t capture the wrap-around artifact that indicates aliasing.