Gastritis: This is often the most common cause of upper abdominal pain, including the left quadrant, due to inflammation of the stomach lining, which can be caused by alcohol, non-steroidal anti-inflammatory drugs (NSAIDs), stress, or infection with Helicobacter pylori.

Peptic Ulcer Disease: Ulcers in the stomach or duodenum can cause pain in the upper abdomen, which may radiate to the left upper quadrant.
Gastroesophageal Reflux Disease (GERD): Although GERD typically causes heartburn, it can also lead to upper abdominal pain.
Pancreatitis: While pancreatitis more commonly causes mid-abdominal pain, it can radiate to the back and upper quadrants.
Splenic Infarction or Rupture: Given the location in the left upper quadrant, issues with the spleen, such as infarction or rupture, should be considered, especially if there's a history of trauma.

Myocardial Infarction: Although less common in a 30-year-old, myocardial infarction can present with epigastric or upper abdominal pain, especially if the inferior wall of the heart is involved.
Pulmonary Embolism: In rare cases, a pulmonary embolism can cause referred pain to the upper abdomen if the embolism affects the lung's lower lobes.
Aortic Dissection: This is a life-threatening condition where the inner layer of the aorta tears, and while it typically presents with severe back or chest pain, it can sometimes cause abdominal pain.

Splenic Artery Aneurysm: A rare condition that can cause left upper quadrant pain if the aneurysm ruptures.
Left Diaphragmatic Hernia: A rare condition where abdominal contents herniate through the diaphragm, potentially causing upper abdominal pain.
Gastric Cancer: Although rare in a 30-year-old, gastric cancer can cause upper abdominal pain and should be considered if risk factors are present or if the pain is persistent and accompanied by other symptoms like weight loss or difficulty swallowing.