This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: All India exam, Subject name: Radiology
Doppler effect results from the change in 
Select and explain. 
A. Amplitude of sound, B. Frequency of sound, C. Direction of sound, D. Intensity of sound
B: Frequency of sound
Answer: B: Frequency of sound 
Explanation: An ultrasound beam insonating a blood vessel is paially reflected by red blood cells. If these are moving, there is a change in the frequency of the reflected pulse: an increase in frequency if the flow is towards the probe and a decrease if it is away from it. This is the Doppler effect and the changes in frequency referred to as the Doppler shift. (Ref: Grainger and Allison&;s diagnostic radiology 6th edition, page 63)