Renal hypoplasia means that the kidney volume is less than 50% of normal, but the development and differentiation of the nephron is normal, and the ureter is normal. Renal dysplasia can be unilateral or bilateral. Unilateral contralateral kidneys often have compensatory hypertrophy. True renal dysplasia is a congenital disease, manifested only by a small kidney but normal renal parenchyma. Segmental renal dysplasia, also known as Ask-Upmark kidney, is characterized by a small kidney and one or more deep grooves on the surface of the kidney. Its lower kidney parenchyma is thyroid-like, and reflux is its possible cause.