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
After a single episode of painless gross hematuria in a boy. Doctor performed an excretory urogram showing a filling defect towards the lower renal infundibulum 1.5 cm. in size. What will be the next investigation to be done - 
Select and explain. 
A. Cystoscopy, B. Urine cytology, C. USG, D. Retrograde pyelography
C: USG
Answer: C: USG 
Explanation: There can be various reason for a filling defect on IVP - stones, mass, cyst etc.
They can be very well be diagnosed by USG.