Tenderness under the right costal arch is the gallbladder area (right hypochondrium, where Murphy's sign is elicited), and an obese woman fits the classic gallstone risk profile (female, fat, forty). So symptomatic gallstones is the best fit.

• Appendicitis and diverticulitis cause lower-abdominal (RIF/LIF) tenderness; pancreatitis gives epigastric pain radiating to the back.