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: Surgery
A 67 year old man presented initially after a single episode of gross painless haematuria. An excretory urogram demonstrated a 1.5 mm round filing defect in the right lower renal infundibulum.The best study to obtain next is - 
Select and explain. 
A. Urinary cytology, B. Cystoscopy, C. Ultrasonography, D. Retrograde pyelography
C: Ultrasonography
Answer: C: Ultrasonography 
Explanation: None