A kidney stone, also known as a renal calculus (from the Latin rēnēs, "kidneys," and calculus, "pebble"), is a solid concretion or crystal aggregation formed in the kidneys from minerals in the urine. Kidney stones typically leave the body by passage in the urine stream, and many stones are formed and passed without causing symptoms. If stones grow to sufficient size (usually at least 3 millimeters (0.12 in)) they can cause blockage of the ureter. This leads to pain, most commonly beginning in the flank or lower back and often radiating to the groin or genitals. This pain is often known as renal colic and typically comes in waves lasting 20 to 60 minutes. Other associated symptoms include: nausea, vomiting, fever, blood in the urine, pus in the urine, and painful urination.The diagnosis of kidney stones is made on the basis of information obtained from the history, physical examination, urinalysis, and radiographic studies. Ultrasound examination and blood tests may also aid in the diagnosis. Urinary stones are typically classified by their location in the kidney (nephrolithiasis), ureter (ureterolithiasis), or bladder (cystolithiasis), or by their chemical composition (calcium-containing, struvite, uric acid, or other compounds). About 80% of those with kidney stones are men. Blockage of the ureter causes decreased kidney function and dilation of the kidney.In those who have previously had stones, prevention is recommended by drinking fluids such that more than two liters of urine is produced per day. If this is not effective enough, thiazide diuretic, citrate or allopurinal may be taken. It is recommended that soft drinks containing phosphoric acid (typically colas) be avoided.When a stone causes no symptoms, watchful waiting is a valid option. For stones which are causing symptoms, pain control is usually the first measure, using medications such as nonsteroidal anti-inflammatory drugs or opioids. More severe cases may require procedures. For example, some stones can be shattered into smaller fragments using extracorporeal shock wave lithotripsy. Some cases require more invasive procedures. Examples of these are cystoscopic procedures such as laser lithotripsy or percutaneous techniques such as percutaneous nephrolithotomy. Sometimes, a tube (ureteral stent) may be placed in the ureter to bypass the obstruction and alleviate the symptoms, as well as to prevent ureteral stricture after ureteroscopic stone removal.