This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Radiology
Distal Acoustic shadow is seen in? 
Select and explain. 
A. Air, B. Calculi, C. Bone, D. All of the above
D: All of the above
Answer: D: All of the above 
Explanation: All of the above REF: Ultrasound Teaching Manual by Matthias Hofer page - 15 In simple terms when sound waves strike an interface which reflects it back, it shows on USG as a hypoechoic or anechoic shadow beyond the structure. It is seen with gas and at interface like bone, calculi and stones.