Gastroesophageal Reflux Disease (GERD): The patient's symptoms of left upper quadrant intermittent pain one hour after eating, accompanied by nausea, are consistent with GERD. The absence of vomiting, fever, and rebound tenderness further supports this diagnosis. The use of Protonix, a proton pump inhibitor, also suggests that GERD is being considered as a primary diagnosis.

Peptic Ulcer Disease: The patient's symptoms of pain after eating and nausea could also be indicative of peptic ulcer disease. The borderline size of the spleen is not directly related to peptic ulcer disease but does not rule it out either.
Functional Dyspepsia: This condition involves recurring signs and symptoms of indigestion that have no obvious cause. The patient's symptoms of pain after eating and nausea without other alarming signs could fit this diagnosis.
Splenic Flexure Syndrome: Although less common, this condition involves intermittent abdominal pain due to gas or fecal material accumulating in the splenic flexure of the colon. The location of the pain and its relation to eating could suggest this diagnosis.

Splenic Infarction or Rupture: Although the patient does not have fever or rebound tenderness, a splenic infarction or rupture could present with left upper quadrant pain. The borderline size of the spleen on ultrasound warrants careful consideration of these conditions, especially if the patient has a history of trauma or conditions predisposing to splenic infarction.
Pancreatitis: Acute pancreatitis can present with abdominal pain that radiates to the back, nausea, and vomiting. Although the patient does not have all these symptoms, pancreatitis is a condition that should not be missed due to its potential severity.
Gastric Cancer: Although less likely in the absence of significant weight loss, early satiety, or other alarming symptoms, gastric cancer can cause abdominal pain and nausea. It is essential to consider this diagnosis, especially in older patients or those with risk factors.

Splenic Artery Aneurysm: This is a rare condition that could cause left upper quadrant abdominal pain if the aneurysm ruptures or is symptomatic. It is more common in women, especially during pregnancy.
Left Upper Quadrant Mass or Tumor: Although rare, a mass or tumor in the left upper quadrant could cause intermittent pain, especially if it involves or compresses nearby structures like the stomach, spleen, or pancreas.
Intestinal Obstruction: An obstruction in the intestines could cause abdominal pain, nausea, and vomiting. However, the symptoms provided do not strongly suggest this diagnosis unless the obstruction is partial or intermittent.