Cholecystitis: The presence of epigastric and right upper quadrant pain along with a positive Murphy sign is highly suggestive of acute cholecystitis. The Murphy sign, which is tenderness in the right upper quadrant that increases with inspiration, is a classic finding in cholecystitis.

Cholelithiasis: While cholelithiasis (gallstones) itself may not cause pain unless it obstructs the cystic duct, it is often associated with cholecystitis and can cause biliary colic, presenting with similar pain patterns.
Peptic Ulcer Disease: Peptic ulcers can cause epigastric pain that may radiate to the right upper quadrant and can sometimes mimic the pain of cholecystitis, although a positive Murphy sign would be less common.
Biliary Dyskinesia: This condition involves abnormal contraction of the gallbladder, which can cause biliary-type pain, often without evidence of gallstones.

Myocardial Infarction: Although less common, myocardial infarction can present with epigastric pain, especially in women, and is a diagnosis that could be deadly if missed.
Pneumonia: Right lower lobe pneumonia can cause right upper quadrant pain due to referred pain from the diaphragm and should be considered, especially if there are respiratory symptoms.
Hepatic Abscess or Hepatitis: These conditions can cause right upper quadrant pain and, while less common, are important to consider due to their potential severity.

Carcinoma of the Gallbladder: This is a rare malignancy that can cause chronic right upper quadrant pain and may mimic cholecystitis.
Sphincter of Oddi Dysfunction: This condition involves abnormal function of the sphincter at the junction of the bile and pancreatic ducts with the duodenum, leading to biliary-type pain.
Porphyria: Acute intermittent porphyria is a rare genetic disorder that can cause severe abdominal pain, among other symptoms, and could potentially be considered in the differential diagnosis of unexplained abdominal pain.