A 60 year old man presents with severe colicky pain from his right flank radiating to his groin

A 60 year old man presents with severe colicky pain from his right flank radiating to his groin. His urinanalysis reveals trace blood cells. What is the single most discriminatory investigation?
A. Ultrasound Scan abdomen
B. X-ray KUB
C. Colonoscopy
D. Upper GI endoscopy
E. Laparoscopy
( the key is A, shouldn’t be B for investing renal stone? ) and What is initial investigation for renal stone?