Formation of, and thus also presence of,Ā [calculi] in theĀ [urinary tract]. urolithiasis isĀ the formation of urinary calculi (āstonesā) in the urinary system. nephrolithiasis or. ākidney stonesā refers to calculi or stones in the kidney and is the most common form of urinary tract. stone disease. Ureter and bladder calculi almost always originate in the kidneys.