Perhaps the most painful problem a person can have with his or her internal plumbing problem is the kidney stone. It's also the most common a person can have with the urinary system.

Men get more kidney stones than women do, and about 12 percent of men can expect a kidney stone by age 70. This problem has a long history — kidney stones have been found in a 7,000-year-old Egyptian mummy.

Also called nephrolithiasis, kidney stones form in your kidney, bladder, or ureter (one of a pair of tubes that carry urine from the kidney to the bladder) where there is a buildup of salt or mineral crystals in the urine. They vary in size, shape, and composition, and they can be smooth or jagged.

There are several types of stones. Calcium stones, the crystallization of calcium salts, make up 75 to 85 percent of all stones and mostly consist of calcium and oxalate. Uric acid stones account for about 8 percent of all cases. Struvite stones are large and rough and form as a result of urinary tract infections. They occur mostly in women.

Recognizing the signs of kidney stones:

When a kidney stone passes out of your body through the ureter, it produces excruciating pain. Fortunately, most stones pass without causing additional problems or serious damage. Here are the common symptoms of kidney stones:

* Excruciating pain in the lower back or the abdomen that travels to the groin area

* Frequent urge to urinate

* Burning sensation during urination

* Bloody urine

On the next page, learn about how kidney stones are diagnosed and treated.