Diverticulitis: Given the location of the pain on the left side of the abdomen and the presence of direct tenderness, diverticulitis is a strong consideration. The absence of changes in bowel habits, fever, or vomiting does not rule out this diagnosis, especially if the diverticulitis is mild or in its early stages.

Irritable Bowel Syndrome (IBS): Although IBS typically presents with alterations in bowel habits, some patients may experience persistent abdominal pain without significant changes in bowel movements. The absence of bloating, nausea, fever, or vomiting does not exclude IBS.
Gastroesophageal Reflux Disease (GERD): Despite the pain not being associated with food intake, GERD can sometimes cause abdominal pain. The presence of a pectus excavatum might increase the likelihood of GERD due to increased intra-abdominal pressure.
Left-sided Inflammatory Bowel Disease (IBD): Conditions like ulcerative colitis or Crohn's disease affecting the left side of the colon could present with left-sided abdominal pain. However, the absence of changes in bowel habits, weight loss, or blood in stool makes this less likely.

Colon Cancer: Although less likely given the patient's age and lack of weight loss, changes in bowel habits, or rectal bleeding, colon cancer must be considered, especially if there's a family history. The presence of a pectus excavatum is not directly related but does not exclude this diagnosis.
Abdominal Aortic Aneurysm (AAA): AAA can present with abdominal pain and is a medical emergency. The pain is often midline but can be felt on the left side. The absence of back pain or a palpable mass does not rule out AAA.
Splenic Infarct or Rupture: Given the left-sided pain, conditions affecting the spleen, such as infarct or rupture, must be considered, especially if there's a history of trauma or underlying conditions like sickle cell disease.

Splenic Artery Aneurysm: A rare condition that could cause left-sided abdominal pain, often associated with a palpable mass or rupture in severe cases.
Pancreatic Neoplasm: Although pancreatic cancer typically presents with weight loss, jaundice, and pain radiating to the back, a tumor in the tail of the pancreas could cause left-sided abdominal pain.
Left Renal Pathology (e.g., Pyelonephritis, Kidney Stone): While these conditions often present with flank pain, severe cases or stones in the lower pole of the kidney could cause abdominal pain that is perceived on the left side.