What is the most common cause of an abdominal mass in newborns?

The most common cause of an abdominal mass in newborns is multicystic dysplastic kidney (MCDK). This condition arises due to improper development of the renal tissue during fetal life, leading to the formation of multiple cysts in place of normal kidney tissue. MCDK is often unilateral, affecting one kidney while the other may be normal or have a different pathology.

In newborns, detection of an abdominal mass can be alarming, and MCDK is typically identified through prenatal imaging or shortly after birth during physical examination or routine imaging studies. Its prevalence and clinical presentation make it a primary consideration when evaluating abdominal masses in this age group.

Other conditions listed, while they may result in abdominal masses, are either far less common or occur later in life. Renal cell carcinoma primarily affects adults. Von Hippel-Lindau disease is hereditary and associated with various tumors but is not a common cause of an abdominal mass in newborns. Angiomyolipoma, while it can occur in children, is also more commonly seen in adults, particularly in association with tuberous sclerosis.