Q. A 4-year-old female presents to your clinic for evaluation of an abdominal mass. The patient's mother discovered a mass in the child's abdomen while bathing her. The mass is not associated with any symptoms such as pain or vomiting. There is no history of fevers or weight loss. The patient is otherwise healthy with no significant past medical history prior to this discovery. She was born at full-term via vaginal delivery with no complications. On physical examination, her blood pressure is 100/60 mm Hg, heart rate 80/min, respiratory rate 25/min, and oxygen saturation is 99 percent on room air. Her head and neck assessment demonstrates no abnormalities. Chest auscultation reveals clear lung sound bilaterally, and her heart is in regular rate and rhythm, with no murmurs, rubs, or gallops. You detect a firm, non-tender, and well-circumscribed mass in her abdomen measuring approximately 8 cm in diameter. The mass is not associated with any hepatosplenomegaly or lymphadenopathy. You order a CT scan of her abdomen (see image). Based on the information you have at this time, which of the following is the most likely diagnosis?