A 74 year old man develops severe, steady, mild abdominal pain approximately 30 minutes after eating. He has lost 12 Kg over the past few months because he wants to avoid the pain after eating. Abdominal examination during an asymptomatic interval is normal. Stool guaiac examination results are negative, as colonoscopy and barium studies. The complete blood count is normal. What is the most likely diagnosis?

**Core Concept**
The patient's symptoms suggest a disorder of the gastrointestinal tract that is associated with pain and weight loss, but the absence of blood, structural abnormalities, and inflammation on diagnostic tests makes it difficult to pinpoint the exact cause. This clinical presentation is often seen in patients with functional gastrointestinal disorders.

**Why the Correct Answer is Right**
The patient's symptoms of postprandial abdominal pain and significant weight loss, despite a normal physical examination and negative diagnostic tests, are characteristic of **Functional Dyspepsia**. This condition is often associated with a hypersensitive stomach, which reacts to normal stimuli, such as eating, with pain and discomfort. The patient's behavior of avoiding eating to alleviate pain has led to significant weight loss.

**Why Each Wrong Option is Incorrect**
**Option A:** **Peptic Ulcer Disease** is unlikely, as the patient's symptoms are not typical (e.g., nocturnal pain, weight loss, and hematemesis are more common), and diagnostic tests (colonoscopy, barium studies) did not show any structural abnormalities.

**Option B:** **Irritable Bowel Syndrome (IBS)** is a possibility, but the patient's symptoms are more focused on postprandial pain and weight loss, which are not as typical of IBS, which often presents with abdominal pain, bloating, and changes in bowel habits.

**Option C:** **Small Intestine Bacterial Overgrowth (SIBO)** can cause postprandial bloating and pain, but the patient's significant weight loss and lack of other symptoms (e.g., diarrhea, fever) make this diagnosis less likely.

**Option D:** **Celiac Disease** is a possibility, as it can cause abdominal pain and weight loss, but the patient's normal complete blood count and lack of other symptoms (e.g., diarrhea, fatigue) make this diagnosis less likely.

**Clinical Pearl / High-Yield Fact**
Functional gastrointestinal disorders, such as functional dyspepsia, are a common cause of abdominal pain and weight loss in older adults, and a thorough diagnostic workup may not always reveal a clear cause. A high index of suspicion and a thorough history are essential in making a diagnosis.

**Correct Answer:** .

✓ Correct Answer: C. Mesenteric angina