Final answer:
Kidney stones are mineral crystals that form in urine and can block the flow of urine out of the body. They typically form when substances like calcium become highly concentrated in the urine.
Step-by-step explanation:
A kidney stone is a solid crystal that forms in a kidney from minerals in the urine. When substances in the urine become highly concentrated, such as calcium, oxalate, and phosphorus, kidney stones can form. If kidney stones block a ureter, they can prevent the flow of urine out of the body and cause severe pain or bleeding. Large kidney stones may need to be removed surgically or shattered with high-intensity ultrasound.