A 32-year-old man from Mumbai with a 3-year history of Crohn disease (ileocolic involvement) on azathioprine and mesalamine presents with a 2-week history of fever (38.5°C), right lower abdominal pain with a palpable mass, and mild diarrhea. Abdominal ultrasound shows a 4 cm × 3 cm hypoechoic mass in the right iliac fossa with surrounding bowel wall thickening. Laboratory findings: Hb 9.2 g/dL, WBC 12,500/μL, CRP 8.5 mg/dL. What is the most likely diagnosis?

Explanation

## Diagnosis: Intra-Abdominal Abscess Secondary to Crohn Disease ### Clinical Presentation This patient has classic features of an intra-abdominal abscess complicating Crohn disease: **Key Point:** The triad of fever, palpable abdominal mass, and localized tenderness in a patient with known Crohn disease is pathognomonic for abscess formation. ### Imaging Findings - **Hypoechoic mass** (fluid collection) - **Surrounding bowel wall thickening** (inflammatory response) - **Right iliac fossa location** (consistent with ileocolic Crohn disease) - **Size 4 × 3 cm** (clinically significant abscess requiring intervention) ### Laboratory Markers - **Elevated CRP (8.5 mg/dL):** Indicates active inflammation/infection - **Leukocytosis (WBC 12,500/μL):** Suggests bacterial infection - **Anemia (Hb 9.2 g/dL):** Chronic inflammation from underlying IBD ### Pathophysiology of Abscess in Crohn Disease ```mermaid flowchart TD A[Crohn disease with transmural inflammation]:::outcome --> B[Perforation of bowel wall]:::action B --> C[Localized peritonitis]:::action C --> D{Walling off by omentum/adjacent bowel?}:::decision D -->|Yes| E[Intra-abdominal abscess]:::outcome D -->|No| F[Generalized peritonitis]:::urgent E --> G[Fever + palpable mass + elevated inflammatory markers]:::outcome ``` **Clinical Pearl:** Abscess formation is one of the most common complications of Crohn disease, occurring in 10–30% of patients. It represents a contained perforation. ### Management of Crohn Abscess **High-Yield:** The standard of care is: 1. **Percutaneous drainage** (CT or ultrasound-guided) — first-line 2. **Broad-spectrum antibiotics** (covering gram-negative and anaerobes) 3. **Bowel rest** and nutritional support 4. **Continuation of immunosuppression** (azathioprine) 5. **Elective surgery** after resolution of acute inflammation (if abscess recurs or cannot be drained) ### Why Not the Other Options? | Feature | Abscess (Correct) | Acute Exacerbation | Lymphoma | Perforated Fistula | | --- | --- | --- | --- | --- | | **Palpable mass** | Yes (fluid) | No | Possible (tumor mass) | No | | **Imaging: hypoechoic fluid** | Yes | No | Heterogeneous mass | Free air/peritoneal fluid | | **Fever pattern** | Persistent, localized | Variable | Low-grade or absent | Acute, severe peritonitis | | **WBC response** | Moderate elevation | Mild–moderate | Normal or low | Marked elevation | | **Management** | Drainage + antibiotics | Medical therapy | Chemotherapy | Emergency surgery | **Warning:** Do not confuse abscess with acute exacerbation. Exacerbation presents with increased diarrhea and cramping but lacks a discrete mass and fever is less prominent. Abscess has a focal, palpable finding and imaging evidence of fluid collection. **Mnemonic for Crohn Complications — "FISTULAS":** - **F**istulas (enterocutaneous, enterovesical, rectovaginal) - **I**ntra-abdominal **abscess** - **S**trictures - **T**oxic megacolon - **U**lcerations (transmural) - **L**ymphoma (increased risk) - **A**nemia - **S**epsis [cite:Harrison 21e Ch 297]