This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Urolithiasis, Subject name: Surgery
Radiolueent kidney stones - 
Select and explain. 
A. Cysteine, B. Uric acid, C. Oxalate, D. Struvite
B: Uric acid
Answer: B: Uric acid 
Explanation: Ans. is 'b' i.e., Uric acid o Lucent stones on plain x-ray KUB are -Uric acidTriametrineCystineXanthineIndinavir stonePure matrix stoneso Almost all stones are opaque on CT scan, varying considerbly in density, but INDINAVIR and PURE MATRIX stones are usually undetectable on CT.o Non contrast helical CT scan is gold standard investigation for urolithiasis now.o CT signs diffemtiating ureteric calculus and pelvic pblebolith are soft tissue rim sign - soft tissue around stone - ureteric calculus.o Tail sign - soft tissue extending towards calcification - phlebolith.