Differential Diagnosis for 45-year-old Male with Diffuse Abdominal Pain and Discomfort

Irritable Bowel Syndrome (IBS): Given the diffuse abdominal pain, discomfort, and nausea without vomiting, IBS is a strong consideration. The burning nature of the pain sometimes can also be associated with IBS, especially if there's an overlap with dyspepsia.

Gastroesophageal Reflux Disease (GERD): The burning nature of the pain could suggest GERD, especially if the pain is located in the upper abdomen or chest and worsens with lying down or eating certain foods.
Peptic Ulcer Disease: Peptic ulcers can cause burning abdominal pain, typically in the upper abdomen, and can be associated with nausea. The absence of vomiting does not rule out this diagnosis.
Functional Dyspepsia: This condition involves upper abdominal discomfort or pain that is not explained by other pathologic conditions. It can present with a variety of symptoms including burning pain, nausea, and discomfort after eating.

Acute Coronary Syndrome: Although less common, cardiac causes of abdominal pain (such as myocardial infarction) can present atypically, especially in diabetic patients or women, but should be considered in any patient with risk factors.
Appendicitis: While the pain of appendicitis typically starts as diffuse and then localizes to the right lower quadrant, atypical presentations can occur, especially in older adults or those with an atypical appendix location.
Mesenteric Ischemia: This is a life-threatening condition that can present with diffuse abdominal pain out of proportion to physical examination findings, and any suspicion warrants immediate investigation.
Pancreatitis: Acute pancreatitis can cause severe, persistent abdominal pain that may be diffuse or localized to the upper abdomen, and is associated with nausea and vomiting.

Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease or ulcerative colitis can cause chronic abdominal pain, diarrhea, and sometimes nausea. While not the first consideration for acute onset, they should be considered in the differential for chronic or recurrent symptoms.
Celiac Disease: An autoimmune reaction to gluten can cause abdominal pain, diarrhea, and malabsorption. It's less likely to present acutely but should be considered in patients with chronic symptoms.
Abdominal Migraine: A condition that can cause recurrent episodes of severe abdominal pain, often accompanied by other migraine symptoms like headache or nausea. It's more common in children but can occur in adults.