Nephrolithiasis is the presence of one or more stones in the kidney. Note that in this specimen the kidney has been cut longitudinally and opened to show the calyces screen the pelvis and the ureter. There is a large staghorn calculus at the top of the screen centrally and another stone in the left upper portion of the kidney.
Note that staghorn calculi are often associated with Proteus mirabilis infection of the kidney. Proteus and other organisms produce a urease which forms ammonia from urea. The ammonia raises the pH of the urine resulting in the formation of struvite stones (magnesium ammonium phosphate stones).