Inborn Errors of Metabolism- the Great Hepatic Masquerader

An 18 year old young man with chronic migraines is referred to hepatology clinic for evaluation of hepatosplenomegaly and thrombocytopenia. Neurology obtained a brain MRI showing calvarial thickening with marrow signaling but otherwise was unremarkable. His AST and ALT are 138 U/L and 150 U/L respectively, platelets 74,000/uL and hemoglobin 10.2 g/dL. At the visit, he complains of chronic leg pain that has been attributed to “growing pains”. He denies alcohol or drug use. On physical exam, liver is palpable 4 cm below costal margin at mid clavicular line with soft contour, spleen is palpable 8 cm below costal margin at mid axillary line. There is tenderness to palpation above the left femur with decrease passive range of motion secondary to pain. What is the best diagnostic test to determine etiology?