A 42-year-old woman presents to the emergency department with a 12-hour history of severe,… — USMLE Step 3

A 42-year-old woman presents to the emergency department with a 12-hour history of severe, constant right upper quadrant abdominal pain. The pain radiates to her right scapula and is associated with nausea and two episodes of non-bilious emesis. Her temperature is $38.2^\circ\text{C}$ ( $100.8^\circ\text{F}$ ), blood pressure is $130/80\text{ mmHg}$ , pulse is $98/\text{min}$ , and respiratory rate is $16/\text{min}$ . Physical examination reveals focal tenderness to palpation in the right upper quadrant with inspiratory arrest upon deep palpation. Laboratory studies show a leukocyte count of $13,000/\text{mm}^3$ with $80\%$ neutrophils. Liver function tests, amylase, and lipase are within normal limits. A right upper quadrant abdominal ultrasound is performed, which demonstrates multiple mobile gallstones and a common bile duct measuring $4\text{ mm}$ in diameter. However, there is no gallbladder wall thickening and no pericholecystic fluid visualized.

Which of the following is the most appropriate next imaging study?

A
Computed tomography (CT) of the abdomen and pelvis with intravenous contrast
B
Magnetic resonance cholangiopancreatography (MRCP)
Hepatobiliary iminodiacetic acid (HIDA) scanAnswer
D
Endoscopic retrograde cholangiopancreatography (ERCP)
E
Repeat right upper quadrant abdominal ultrasound in 48 hours

Answer

Hepatobiliary iminodiacetic acid (HIDA) scan

The correct answer is to order a hepatobiliary iminodiacetic acid (HIDA) scan, also known as cholescintigraphy. This patient presents with a classic clinical picture of acute cholecystitis, including right upper quadrant pain, fever, leukocytosis, and a positive Murphy sign. Right upper quadrant ultrasound is the standard initial imaging modality. While the ultrasound confirmed the presence of gallstones, it did not show definitive signs of acute inflammation (e.g., gallbladder wall thickening >4 mm, pericholecystic fluid). When the clinical suspicion is high but the initial ultrasound is equivocal, a HIDA scan is the most sensitive and specific next step. In a positive HIDA scan, the radiotracer enters the liver and common bile duct but fails to enter the gallbladder due to cystic duct obstruction, which is the pathophysiologic hallmark of acute cholecystitis.

Step-by-Step Solution

Analyze the clinical presentation

The patient has signs and symptoms highly suspicious for acute cholecystitis (fever, leukocytosis, right upper quadrant pain, positive Murphy sign).

Establishing the pre-test probability is crucial before selecting diagnostic imaging.

Evaluate the initial imaging results

Right upper quadrant ultrasound shows gallstones but lacks definitive findings of acute cholecystitis (no wall thickening, no pericholecystic fluid).

Ultrasound is the best initial test, but its sensitivity can vary; lack of secondary signs makes it equivocal.

Select the appropriate follow-up test

A cholescintigraphy (HIDA) scan is selected to definitively confirm or rule out acute cholecystitis.

HIDA scan evaluates gallbladder function and cystic duct patency; non-visualization of the gallbladder confirms the diagnosis.

Key Concept

When a patient has a clinical presentation highly suspicious for acute cholecystitis but the initial right upper quadrant ultrasound is equivocal, cholescintigraphy (HIDA scan) is the most appropriate next diagnostic step.

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