Gastritis: This is often the most common cause of upper left quadrant abdominal pain due to the stomach's location. Gastritis can be caused by various factors including infection with Helicobacter pylori, long-term use of nonsteroidal anti-inflammatory drugs (NSAIDs), high alcohol consumption, stress, and autoimmune disorders.

Peptic Ulcer Disease: Ulcers in the stomach or duodenum can cause pain in the upper abdomen, which may be localized to the left upper quadrant. Symptoms can include burning stomach pain, bloating, and nausea.
Gastroesophageal Reflux Disease (GERD): Although GERD typically causes symptoms in the chest and upper central abdomen, it can sometimes radiate to the left upper quadrant. Symptoms include heartburn, regurgitation, and difficulty swallowing.
Pancreatitis: While pancreatitis often causes mid-abdominal pain that radiates to the back, it can sometimes be felt in the left upper quadrant, especially if the tail of the pancreas is involved. Symptoms include severe abdominal pain, nausea, vomiting, and fever.
Splenic Infarction or Rupture: Conditions affecting the spleen, such as infarction or rupture, can cause severe pain in the left upper quadrant. This is often accompanied by symptoms like nausea, vomiting, and left shoulder pain due to referred pain.

Myocardial Infarction: Although myocardial infarction (heart attack) typically presents with chest pain, it can sometimes cause referred pain in the abdomen, including the left upper quadrant. It's crucial to consider this diagnosis, especially in patients with risk factors for coronary artery disease.
Pulmonary Embolism: Similar to myocardial infarction, pulmonary embolism can cause referred pain in the abdomen if the embolism affects the lung's lower lobes. Symptoms can include sudden onset of shortness of breath, chest pain, and abdominal pain.
Aortic Dissection: This is a life-threatening condition where the inner layer of the aorta tears. It can cause severe, tearing pain that may start in the chest and radiate to the abdomen. Immediate diagnosis and treatment are critical.

Splenic Artery Aneurysm: A rare condition where an aneurysm forms in the splenic artery, which can cause left upper quadrant abdominal pain if it ruptures.
Left Diaphragmatic Hernia: A rare congenital or acquired condition where abdominal contents herniate through the diaphragm, potentially causing left upper quadrant pain.
Gastric Cancer: Although rare, gastric cancer can cause chronic pain in the upper abdomen, including the left upper quadrant, along with weight loss, nausea, and vomiting.