Comprehensive and Detailed Explanation From Exact Extract:
In color Doppler, frequency shifts can be displayed either above or below the baseline in spectral Doppler. In spectral Doppler, a positive shift is displayed above the baseline by default (unless inverted). In color Doppler, red/blue assignments depend on color map orientation.
Principles and Instrumentation state:
"A positive Doppler shift (flow toward the transducer) is displayed above the baseline in spectral Doppler."
* Red color (B) does not always correspond to positive shift; it depends on color map.
* Negative shift (C) does not always indicate venous flow.
* Wall filter settings (D) affect low-velocity signals, not shift direction.
Therefore, the correct answer is A: A positive shift is always above the baseline.
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