Hepatoblastoma

A 2-year-old boy presents with a progressively enlarging painless abdominal mass in the right upper quadrant for six weeks, along with failure to thrive, pallor, and low-grade fever. He was born extremely premature with low birth weight. On examination, he has a firm hepatomegaly with a discrete mass palpable below the right costal margin, and phenotypic features of Beckwith-Wiedemann syndrome. Contrast-enhanced CT shows a large solitary heterogeneous hypodense mass in the right lobe with peripheral rim enhancement, internal septations, and intratumoral calcifications. The lesion marked **C** in the diagram is suspected. Which of the following laboratory findings would be MOST CHARACTERISTIC and useful for monitoring treatment response?