What is the most likely diagnosis for a 43-year-old woman with sudden onset of right upper quadrant pain, fever, and a positive Murphy's sign?

The most likely diagnosis in the case presented is acute cholecystitis. This condition is characterized by inflammation of the gallbladder, and the symptoms described are classic signs that can lead to its identification.

The sudden onset of right upper quadrant pain is a hallmark symptom of gallbladder issues, particularly when the pain is intense and localized. A positive Murphy's sign, which occurs when the patient experiences pain upon palpation of the right upper quadrant during inspiration, further supports the diagnosis of acute cholecystitis. The presence of fever suggests an inflammatory process, which aligns with the infection or inflammation commonly seen in acute cholecystitis.

In contrast, diverticulitis generally presents with lower abdominal pain, often on the left side, and is not typically associated with a positive Murphy's sign. Gastroenteritis usually involves diffuse abdominal pain, diarrhea, and vomiting rather than local tenderness and fever isolated to the right upper quadrant. Appendicitis primarily presents with right lower quadrant pain, nausea, and possibly fever, making it less likely in this scenario.

Thus, the clinical presentation of right upper quadrant pain, fever, and a positive Murphy's sign strongly indicates acute cholecystitis as the most likely diagnosis.