A 3-year-old child presents with fever, anemia, and a palpable mass in the right upper quadrant of the abdomen. During the abdominal examination, the NP identifies mild pain on palpation. A urine specimen shows hematuria. The child's body temperature s 100°F. The most likely diagnosis is:

A. urinary tract infection (UTI).
B. pediatric functional abdominal pain.
C. a Wilms tumor.
D. vesicoureteral reflux.