- A newborn boy presented with a large, firm left flank mass found on physical examination. An ultrasound revealed a solid renal mass. - The most likely diagnosis is congenital mesoblastic nephroma (CMN), a rare renal tumor that typically presents in infants less than 3 months old as a painless abdominal or flank mass. - CMN is usually benign and curable with surgical excision alone, though cellular variants and incomplete resection can cause more aggressive behavior.