Answer is 'b' i.e. Uric acid stones are dropped from above It is the oxalate stones that are usually dropped from above (i.e. formed in the kidney and dropped in the bladder through ureter). Uric acid stones are mostly formed within the bladder. In general most of the bladder calculi are formed within the bladder itself. Only some stones are formed in the kidney and pass into the bladder, where they grow by additional deposition of crystals. Etiology for Bladder calculi Bladder outlet obstruction is the most common cause of vesical calculi in adults. Other causes are - neurogenic bladder leading to urinary stasis - a foreign body eg. Foley catheter and forgotten DJ stent, which serve as nidi for stones. - bladder diveicula Bladder stones are commonly seen in older men (>50 yrs.) usually because of bladder outlet obstruction In the Western world bladder stones are rare in pediatric population but bladder stones are endemic in children in underdeveloped nations (in Noh Africa, Thailand, Burma, Indonesia i.e. found near the tropics). This is because of low-protein, high carbohydrate diet and chronic dehdyration. These are usually ammonium urate stones. Common vesical stones are Uric acid stones & Struvite stones Other less common stone types are Oxalate stones Cystein stones Struvite stones are also known as 'Jack stones' because of their rough surface as against the smooth surface of uric acid stones. Struvite stones are composed of ammonium, magnesium and calcium phosphates and occurs in urine infected with urea-splitting organisms, eg. proteus. Bladder stones are usually solitary but may also be multiple.