Final answer:
The most likely diagnosis for a 9-month-old boy with a right abdominal mass and no definite right kidney seen on ultrasound is congenital absence of the right kidney.
Step-by-step explanation:
The most likely diagnosis for a 9-month-old boy with a right abdominal mass and no definite right kidney seen on ultrasound is congenital absence of the right kidney. This condition, also known as unilateral renal agenesis, occurs when one kidney fails to develop during fetal development. The absence of the right kidney can result in fluid accumulation and damage to the liver and blood vessels.