Gallbladder disease (cholelithiasis or cholecystitis): This is the most common cause of right upper quadrant pain, especially if it's intermittent and worsens after fatty meals. The pain is often colicky in nature, meaning it comes and goes.

Peptic ulcer disease: Ulcers in the stomach or duodenum can cause pain in the right upper quadrant, especially if the ulcer is located near the pylorus or if there is associated inflammation.
Hepatitis: Inflammation of the liver can cause right upper quadrant pain, and it can be intermittent, especially in the early stages of the disease.
Right kidney stone or pyelonephritis: While kidney stones or infections can cause flank pain, they can also radiate to the right upper quadrant, especially if the stone is moving or if the infection is severe.

Appendicitis (if the appendix is located in a retrocecal position): Although appendicitis typically presents with pain in the lower right quadrant, a retrocecal appendix can cause pain in the right upper quadrant. Missing this diagnosis can lead to perforation and severe complications.
Hepatic artery aneurysm or rupture: A rare but potentially life-threatening condition that requires immediate intervention. The pain can be sudden and severe.
Cholangitis: Infection of the bile ducts, which can be life-threatening if not promptly treated. It often presents with jaundice, fever, and right upper quadrant pain.

Liver abscess: A collection of pus in the liver, which can cause right upper quadrant pain, fever, and weight loss. It's more common in people with certain underlying conditions, such as diabetes or a history of liver disease.
Hepatocellular carcinoma: A type of liver cancer that can cause right upper quadrant pain, especially if the tumor is large or has ruptured.
Right adrenal gland tumor: Tumors of the adrenal gland, such as pheochromocytoma or adrenal carcinoma, can cause right upper quadrant pain, although this is extremely rare.