Gastroesophageal Reflux Disease (GERD): This condition is characterized by stomach acid frequently flowing back into the tube connecting the mouth and stomach (esophagus). Symptoms can include epigastric fullness and back pain due to the reflux of acid. It's a common condition that can cause these symptoms, making it a likely initial consideration.

Peptic Ulcer Disease: This includes both gastric and duodenal ulcers, which can cause epigastric pain and, if the ulcer penetrates through the stomach or duodenal wall, can lead to back pain due to peritoneal irritation.
Cholecystitis: Inflammation of the gallbladder, often due to gallstones, can cause epigastric pain that may radiate to the back.
Pancreatitis: Inflammation of the pancreas can cause severe epigastric pain that radiates to the back, nausea, vomiting, and abdominal tenderness.

Myocardial Infarction: Although less common, myocardial infarction (heart attack) can present with epigastric discomfort or back pain, especially in women or diabetic patients, making it crucial not to miss this potentially life-threatening condition.
Aortic Dissection: A tear in the aorta's inner layer can cause severe, tearing chest or back pain and is a medical emergency.
Pulmonary Embolism: While more commonly associated with respiratory symptoms, a pulmonary embolism can occasionally present with chest or back pain and must be considered due to its high mortality rate if untreated.

Splenic Infarction: Although rare, an infarction of the spleen can cause severe left upper quadrant or epigastric pain that may radiate to the back.
Esophageal Rupture: A spontaneous or traumatic rupture of the esophagus can cause severe chest or back pain and is a surgical emergency.
Tumor of the Pancreas or Stomach: While less common, tumors in these areas can cause epigastric fullness and back pain due to local invasion or metastasis.