**Question:** A renal mass seen on prenatal checkup is most probably

A. Renal cell carcinoma
B. Umbilical hernia
C. Nephrogenic adenoma
D. Congenital cystic disease of the kidney

**Correct Answer:** D. Congenital cystic disease of the kidney

**Core Concept:** Prenatal ultrasound is a crucial tool in detecting abnormalities in the developing fetus, including renal masses. Renal masses can be due to various causes.

**Why the Correct Answer is Right:** Congenital cystic disease of the kidney (CCDK) is a group of rare genetic disorders characterized by the presence of multiple cysts in one or both kidneys. It is the most common cause of a renal mass in fetuses undergoing prenatal ultrasound. CCDK is usually diagnosed prenatally because of the large number of cysts, which can lead to enlarged kidneys and potentially cause complications during birth or later in life.

**Why Each Wrong Option is Incorrect:**

A. Renal cell carcinoma (RCC) is a malignant tumor that develops in the renal parenchyma, not as a mass of multiple cysts. RCC is typically seen in older adults and not commonly detected in fetuses during prenatal ultrasound.

B. Umbilical hernia is a separate congenital anomaly unrelated to renal masses. It involves the herniation of abdominal contents through a defect in the umbilicus.

C. Nephrogenic adenoma is a benign tumor of the kidney, typically seen in adults, not as a renal mass in fetuses. Nephrogenic adenoma is characterized by the proliferation of glandular tissue in the renal parenchyma, which does not result in a mass of multiple cysts.

**Clinical Pearl:** When a renal mass is identified in a fetus during prenatal ultrasound, it is essential to investigate further for conditions like CCDK, which can have significant long-term implications for the child and its family. Early diagnosis allows for proper management and surveillance, potentially preventing severe complications like obstructive uropathy, hypertension, and end-stage renal disease in later life.