A 45-year-old female presents to the emergency department with a 24-hour history of sharp, right upper quadrant abdominal pain radiating to her right shoulder, nausea, and fever. Her vitals are: BP 135/85 mmHg, HR 102 bpm, RR 19/min, Temp 101.3°F (38.5°C), and SpO2 97% on room air. On examination, you note a positive Murphy's sign. Laboratory results show a mild leukocytosis. Suspecting cholecystitis, what is the most appropriate initial diagnostic test to order?
Abdominal CT scan
Magnetic resonance cholangiopancreatography (MRCP)
Hepatobiliary (HIDA) scan
Right upper quadrant ultrasound