RADIOLOGY: KIDNEY: Case# 33654: MULTICYSTIC DYSPLASTIC KIDNEY. 6 month old male with abnormal prenatal ultrasound and flank mass. Ultrasound over the palpable mass reveals multiple noncommunicating cysts that occupy the entire renal fossa with no evidence of renal parenchyma. The cysts are variable in size with the largest cysts located in the periphery. Most infants with a multicystic dysplastic kidney will present with a large palpable flank mass. The majority of MCDK s are unilateral and represent the most common cause of abdominal masses in infancy. If bilateral MCDK is present the findings are lethal. Approximately 15-15% of patients will have an abnormality of the contralateral kidney. The differentiation of MCDK from hydronephrosis is essential since treatment options vary. Nuclear medicine studies should be performed to assess function. Typically, on follow up ultrasound the MCDK shrinks and eventually may disappear.