Ketoacidosis is when the body has abnormally high concentrations of ketones. This lowers the pH of the blood. Ketoacidosis can cause urine to smell bad. The smell is usually described as "like fruit" or "like nail polish remover". The odor is caused by the release of acetone.
Ketoacidosis

Ketoacidosis is a state of extreme ketosis. Ketosis is when the body uses fat for energy instead of sugar. It is normal for a person to be in ketosis sometimes, but in Ketoacidosis this normal biological state is out of control and is dangerous.

Ketoacidosis occurs most often in those with uncontrolled type 1 diabetes. People who have type 1 diabetes do not produce insulin to regulate the ketones in their blood. This makes their blood more acidic, which is compounded by the high blood sugar that also results from a lack of insulin.

Alcoholism can also be a cause of ketoacidosis.
Ketoacidosis is a medical emergency. If you are having symptoms you believe might be caused by ketoacidosis, you should seek medical care immediately.