**Question:** A 39-year-old male presents with bloody diarrhea. Multiple stool examinations fail to reveal any ova or parasites. A barium examination of the patient's colon reveals a characteristic "string sign." A colonoscopy reveals the rectum and sigmoid poins of the colon to be unremarkable. A biopsy from the terminal ileum reveals numerous acute and chronic inflammatory cells within the lamina propria. Worsening of the patient's symptoms results in emergency resection of the distal small intestines. Gross examination of this resected bowel reveals deep, long mucosal fissures extending deep into the muscle wall. Several transmural fistulas are also found. What is the best diagnosis for this patient?

A. Crohn's Disease
B. Colitis
C. Ulcerative Colitis
D. Fecal impaction

**Correct Answer:** A. Crohn's Disease

**Core Concept:** Crohn's disease is a type of inflammatory bowel disease (IBD) that affects the entire gastrointestinal tract, particularly the terminal ileum and colon. It is characterized by transmural inflammation, which involves the entire intestinal wall, including the submucosa, muscularis propria, and serosa. In contrast, ulcerative colitis affects the colon and rectum, not the terminal ileum.

**Why the Correct Answer is Right:**

The correct answer is Crohn's disease (option A) because:

1. The patient's bloody diarrhea, mucosal fissures extending into the muscle wall, and transmural inflammation are consistent with Crohn's disease.
2. The other options do not explain these findings:

- Colitis (option B) typically refers to inflammation of the colon and rectum, not involving the terminal ileum.
- Ulcerative colitis (option C) affects the colon and rectum, not the terminal ileum.
- Fecal impaction (option D) is unrelated to the described clinical findings and histological features.

**Why Each Wrong Option is Incorrect:**

Option B (Colitis): Inflammatory bowel disease (IBD) can affect any part of the gastrointestinal tract. However, colitis specifically refers to inflammation of the colon and rectum, which is different from the described clinical and histological findings in this case.

Option C (Ulcerative colitis): Ulcerative colitis is another type of IBD that primarily affects the colon and rectum. Unlike the presented case, ulcerative colitis does not involve the terminal ileum, and thus is not the correct diagnosis.

Option D (Fecal impaction): Fecal impaction is a condition caused by an obstruction due to stool accumulation, not related to the described findings in this case.

**Clinical Pearl:** In clinical practice, it is crucial to differentiate between Crohn's disease and ulcerative colitis, as they have distinct clinical presentations and management strategies. In this case, the patient's findings are consistent with Crohn's disease, affecting the terminal ileum and presenting with mucosal fissures