<s>[INST] context: The diagnostic approach to ureteric colic has changed due to the introduction of new radiological imaging such as non-contrast CT. The role of intravenous urography, which is regarded as the gold standard for the diagnosis of ureteric colic, is being challenged by CT, which has become the first-line investigation in a number of centres.CT has the following advantages over IVU: it has higher sensitivity and specificity for calculus detection, it does not use intravenous contrast medium, it permits alternative diagnoses, and requires a shoer examination time.CT can visualise all radiopaque stones, as well as radiolucent stones such as uric acid and cystine calculi. An impoant limitation of CT is the fact that it does not permit functional evaluation of the kidneys and it is unable to assess the degree of obstruction. Another disadvantage is higher radiation exposure of the patient compared with KUB or IVU. First line investigation for flank pain:-USG First line investigation for renal stone:-x-RAY KUB. ref :- Bailey and Love 27th ed , chapter 76 , pg no :- 1406 Campbell - Walsh urology 11th ed , chapter 52 , pg no 1202 [/INST] question: Best investigation for ureteric stones is ? | options: A: USG B: CT C: CECT D: IVP | answer: B </s>