A 45-year-old woman presents with severe right upper quadrant pain radiating to the back, jaundice, and nausea. She reports dark urine and pale stools over the past two days. Physical examination reveals tenderness in the right upper quadrant. Lab tests show elevated alkaline phosphatase (ALP) , gamma-glutamyl transferase ( GGT), and conjugated bilirubin. What is the most likely initial imaging modality to confirm the diagnosis?
