Gallbladder disease (cholecystitis or cholelithiasis): This is the most common cause of right upper quadrant (RUQ) pain, especially in females. The pain is often postprandial, worsening after fatty meals, and may radiate to the right shoulder.

Peptic ulcer disease: Gastric or duodenal ulcers can cause RUQ pain, especially if the ulcer is located in the duodenum. The pain is often described as burning and may be relieved by eating or worsened by hunger.
Hepatitis: Inflammation of the liver can cause RUQ pain, jaundice, and elevated liver enzymes. The pain is often dull and aching.
Pyelonephritis or kidney stones: Infection or obstruction of the right kidney can cause severe RUQ pain, often radiating to the flank or groin. The pain may be accompanied by fever, nausea, and vomiting.

Appendicitis: Although appendicitis typically presents with lower abdominal pain, it can occasionally cause RUQ pain, especially if the appendix is retrocecal. Missing this diagnosis can lead to perforation and peritonitis.
Hepatic abscess or tumor: A liver abscess or tumor can cause RUQ pain, fever, and weight loss. These conditions require prompt diagnosis and treatment to prevent serious complications.
Cholangitis: Infection of the bile ducts can cause RUQ pain, jaundice, and sepsis. This condition requires urgent diagnosis and treatment to prevent mortality.

Pneumonia or pulmonary embolism: Right lower lobe pneumonia or pulmonary embolism can cause referred pain to the RUQ. These conditions are less common causes of RUQ pain but should be considered in patients with respiratory symptoms.
Intestinal obstruction or volvulus: Obstruction or twisting of the small intestine can cause RUQ pain, nausea, and vomiting. These conditions are rare but require prompt surgical intervention to prevent complications.
Right adrenal gland pathology: Tumors or hemorrhage of the right adrenal gland can cause RUQ pain, although this is a rare occurrence.