Paediatricscommon
A 3-year-old girl is brought in after her mother felt a lump in her tummy while bathing her. She is thriving, plays normally, and has had no fever, weight loss or bone pain; her bowels open daily. Blood pressure is 116/74 mmHg, above the 95th centile for her age and height. A smooth firm mass fills the left flank; it is ballotable and does not extend across the midline. Urinalysis shows blood ++ and no protein. Which one of the following best explains this mass?