A 35-year-old man with a 10-year history of type 2 diabetes mellitus presents with recurrent episodes of severe hypoglycemia despite dietary modifications and reduced insulin doses. His fasting blood glucose is 85 mg/dL, but he experiences symptomatic hypoglycemia (tremor, diaphoresis, confusion) within 2–3 hours of meals. Physical examination reveals a palpable abdominal mass. Which investigation is most appropriate to confirm the suspected diagnosis?