A 52-year-old man is evaluated in the hospital for worsening abdominal pain and new-onset … — USMLE Step 3

A $52$ -year-old man is evaluated in the hospital for worsening abdominal pain and new-onset fever. He was admitted $3$ days ago for acute pancreatitis secondary to gallstones. Initial management with aggressive intravenous hydration and analgesics led to transient improvement. Today, his temperature is $39.2^{\circ}\text{C}$ ( $102.6^{\circ}\text{F}$ ), blood pressure is $110/70\text{ mmHg}$ , pulse is $115\text{/min}$ , and respiratory rate is $20\text{/min}$ . Physical examination reveals severe tenderness in the epigastrium with guarding. A contrast-enhanced computed tomography (CT) scan of the abdomen is obtained. The images demonstrate an acute, heterogeneous collection of fluid and necrotic tissue involving the pancreatic body and tail, with multiple small foci of gas noted entirely within the collection. No well-defined capsule is visualized. Which of the following is the most accurate interpretation of these radiologic findings?

Infected acute necrotic collectionAnswer
B
Pancreatic pseudocyst
C
Acute peripancreatic fluid collection
D
Walled-off necrosis
E
Bowel perforation

Answer

Infected acute necrotic collection

The correct answer accurately synthesizes the temporal course and imaging findings. According to the revised Atlanta classification of acute pancreatitis, an acute necrotic collection occurs within the first

4

weeks and contains both fluid and necrotic solid debris without a well-defined wall. The presence of gas within this collection on CT is a pathognomonic radiologic sign of infection by gas-forming organisms, leading to the definitive diagnosis of an infected acute necrotic collection. This directly correlates with the patient's new-onset fever and clinical deterioration.

Step-by-Step Solution

Analyze the temporal course of the patient's acute pancreatitis.

The patient is

3

days into the illness, which restricts the differential diagnosis to acute-phase complications (less than

4

weeks duration).

Pancreatic collections are strictly classified by their age under the revised Atlanta classification.

Evaluate the specific composition and morphology of the lesion on the CT scan.

The collection is heterogeneous (tissue and fluid) and lacks a capsule.

This differentiates an acute necrotic collection from a simple fluid collection (homogeneous fluid) or a late complication (encapsulated).

Interpret the presence of gas within the collection.

Gas within the necrotic tissue signifies the presence of gas-forming bacteria.

This radiologic finding is highly specific for infection, explaining the patient's clinical deterioration and fever.

Key Concept

Interpretation of CT imaging in acute pancreatitis (Atlanta classification), specifically identifying infected necrosis.

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