Kaplan USMLE Step 2: 3-year-old patient with an abdominal mass

If you’re preparing for the United States Medical Licensing Examination® (USMLE®) Step 2 exam, you might want to know which questions are most often missed by test-prep takers. Check out this example from Kaplan Medical, and read an expert explanation of the answer. Also check out all posts in this series.

A 3-year-old boy is brought to the physician for a well-child visit. The child has no history of serious illness and takes no medications. His immunizations are up to date. He is playful and responds appropriately to conversation and interaction. Physical examination shows an abdominal mass that extends 10 cm below the left costal margin. The remainder of the examination shows no abnormalities. Laboratory studies show the following:

• White blood cells: 7,800/mm³
• Hematocrit: 45%
• Platelets: 289,000/mm³
• INR: 1.0
• Partial thromboplastin time: 17 seconds

A CT scan of the abdomen shows a 9 cm mass encasing the aorta and displacing the left kidney inferiorly. An iodine-131-meta-iodobenzylguanidine (MIBG) scan shows increased uptake by the mass. Which of the following is the most likely diagnosis?

A. Adrenal adenocarcinoma

B. Lymphoma

C. Neuroblastoma

D. Retroperitoneal sarcoma

E. Wilms tumor

The correct answer is C.

Neuroblastoma is the most common solid tumor in children and frequently presents within the first few years of life. Screening tests include urine catecholamines and MIBG scans with confirmation made by surgical pathology. 

Choice A. Adrenal adenocarcinoma will not have an increase in uptake on MIBG scan. Adrenal adenocarcinoma has two peaks in age of presentation: before the age of five, and between 40 to 60 years of age.