What does a one-week-old male infant with a left flank mass, normal right kidney, and numerous noncommunicating round cystic structures in the left renal fossa most probably have?

The presence of numerous noncommunicating round cystic structures in the left renal fossa of a one-week-old male infant is highly indicative of multicystic kidney disease. In this condition, one of the kidneys develops multiple cysts that do not communicate with each other, leading to a left flank mass that can be palpated or visualized via imaging studies.

In the case of multicystic dysplastic kidney, the affected kidney typically appears enlarged due to these cysts, while the contralateral kidney, in this scenario the normal right kidney, maintains its size and function. This finding aligns with the description provided in the question.

Furthermore, severe hydronephrosis would typically present with dilatation of the renal pelvis and possibly the calyces, but not primarily with multiple independent cystic structures. Polycystic kidney disease affects both kidneys and does not result in noncommunicating cysts in a single fossa. Nephroblastoma, or Wilms' tumor, generally presents as a solid mass rather than multiple cystic formations, particularly in infants.

Therefore, the characteristics noted in the question strongly support the diagnosis of a multicystic kidney, making it the most probable diagnosis for this infant.