What is the abnormal condition characterized by excess fluid in the kidney?

The abnormal condition characterized by excess fluid in the kidney is hydronephrosis. This condition typically results from an obstruction that prevents urine from flowing out of the kidney, causing urine to accumulate and lead to swelling or dilation of the renal pelvis and calyces.

Hydronephrosis can occur due to various factors, such as kidney stones, tumors, or structural abnormalities, which may block the ureters or urinary tract. The accumulation of fluid puts pressure on kidney tissue, potentially leading to kidney damage if not addressed.

Understanding hydronephrosis is crucial in pathology and clinical settings, as it directly affects kidney function and can lead to serious complications if the underlying cause is not treated. Other conditions listed, like hydrocephalus, nephropathy, and urolithiasis, do not relate specifically to the excess fluid condition in the kidneys, making hydronephrosis the clear focus for this question.