A 28-year-old woman, gravida 1, para 0, at 26 weeks gestation, presents to the emergency d… — USMLE Step 3

A $28$ -year-old woman, gravida $1$ , para $0$ , at $26$ weeks gestation, presents to the emergency department with a $12$ -hour history of progressively worsening right-sided abdominal pain. She also reports nausea and $2$ episodes of vomiting. Her temperature is $38.2^\circ\text{C}$ ( $100.8^\circ\text{F}$ ), blood pressure is $112/70\text{ mmHg}$ , and heart rate is $108/\text{min}$ . Physical examination reveals localized tenderness to palpation in the right mid-to-upper quadrant of the abdomen, with mild guarding. Laboratory studies show a leukocyte count of $16,500/\text{mm}^3$ . A graded-compression transabdominal ultrasound is performed, which shows a normal right kidney and ovary but fails to visualize the appendix. Clinical suspicion for acute appendicitis remains high. Which of the following is the most appropriate next step in the diagnostic imaging evaluation?

Magnetic resonance imaging of the abdomen and pelvis without contrastAnswer
B
Magnetic resonance imaging of the abdomen and pelvis with gadolinium contrast
C
Computed tomography of the abdomen and pelvis with intravenous contrast
D
Hepatobiliary iminodiacetic acid (HIDA) scan
E
Transvaginal ultrasonography

Answer

Magnetic resonance imaging of the abdomen and pelvis without contrast

The correct answer is magnetic resonance imaging (MRI) of the abdomen and pelvis without contrast. In pregnant patients with suspected acute appendicitis, the initial imaging modality of choice is graded-compression transabdominal ultrasonography. If ultrasonography is non-diagnostic (which frequently occurs in the second and third trimesters because the gravid uterus displaces the cecum and appendix upward), an MRI without contrast is the recommended next step. MRI provides excellent soft-tissue visualization and is highly sensitive and specific for diagnosing appendicitis while avoiding fetal exposure to ionizing radiation.

Step-by-Step Solution

Analyze the clinical presentation and physical exam findings.

The patient is a pregnant woman at

26

weeks gestation with right mid-to-upper quadrant pain, fever, and leukocytosis, which is highly suspicious for acute appendicitis due to the upward displacement of the appendix by the gravid uterus.

Recognizing the anatomic changes of pregnancy is crucial for formulating an accurate differential diagnosis.

Evaluate the results of the initial diagnostic workup.

A transabdominal ultrasound was performed but was non-diagnostic (appendix not visualized).

Ultrasound is the first-line imaging modality for suspected appendicitis in pregnancy, but it has a high rate of non-visualization, especially in the second and third trimesters.

Determine the most appropriate next imaging step.

MRI of the abdomen and pelvis without contrast is the study of choice.

MRI provides high sensitivity and specificity for appendicitis without exposing the fetus to ionizing radiation (unlike CT) or contraindicated contrast agents like gadolinium.

Key Concept

In pregnant patients with suspected appendicitis and a non-diagnostic initial ultrasound, magnetic resonance imaging (MRI) without contrast is the most appropriate next step in diagnostic imaging.

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