Ultrasound (USG) abdomen is the most appropriate initial imaging for acute right upper quadrant pain with fever because it is rapid, non-invasive, and highly sensitive for detecting gallbladder pathology such as cholecystitis or biliary obstruction. USG effectively identifies gallstones, gallbladder wall thickening, and pericholecystic fluid, which are key signs in acute cholecystitis. It also avoids radiation exposure and can be performed bedside, making it ideal for unstable patients. CT or MRI may be used later if USG findings are inconclusive. Abdominal X-ray has limited utility in this context as it does not provide sufficient visualization of hepatobiliary structures.

Reference: Sabiston Textbook of Surgery, Townsend CM et al., 21st Edition.