A 29-year-old woman, gravida 2, para 1, at 26 weeks gestation, is evaluated in the emergen… — USMLE Step 3

A 29-year-old woman, gravida 2, para 1, at 26 weeks gestation, is evaluated in the emergency department for severe, colicky right flank pain that began 4 hours ago. The pain radiates to her right labia. She reports nausea and one episode of emesis, as well as seeing blood in her urine. Temperature is $37.1^\circ\text{C}$ ( $98.8^\circ\text{F}$ ), blood pressure is $135/85 \text{ mm Hg}$ , pulse is $105/\text{min}$ , and respiratory rate is $18/\text{min}$ . Physical examination reveals marked right costovertebral angle tenderness. Fetal heart rate is $145/\text{min}$ and reactive. Urinalysis shows gross hematuria and no leukocyte esterase or nitrites. Renal ultrasonography demonstrates moderate right-sided hydronephrosis without a visible calculus or ureteral jet on the right. Intravenous hydration and pain management with acetaminophen and opioids are initiated, but her pain remains excruciating and she requests further intervention. Her medical history is notable for a childhood metalworking accident that left a microscopic ferromagnetic foreign body in her left orbit, which she has never had surgically removed. Which of the following is the most appropriate next step in management?

A
Magnetic resonance urography without intravenous contrast
B
Diagnostic cystoscopy with retrograde pyelography
Low-dose non-contrast computed tomography of the abdomen and pelvisAnswer
D
Standard-dose computed tomography of the abdomen and pelvis with intravenous contrast
E
Discharge home with oral analgesics and outpatient renal ultrasonography in 48 hours

Answer

The correct answer is low-dose non-contrast computed tomography of the abdomen and pelvis.

Low-dose non-contrast computed tomography of the abdomen and pelvis is the most appropriate next step. The diagnostic algorithm for suspected nephrolithiasis in pregnancy begins with renal ultrasonography. Because mild-to-moderate right-sided hydronephrosis is a normal physiologic finding in pregnancy (due to dextrorotation of the uterus and progesterone-induced ureteral relaxation), ultrasound is often indeterminate. When symptoms persist and definitive diagnosis is required to guide intervention, magnetic resonance (MR) urography without gadolinium is the preferred second-line test. However, this patient has a retained intraocular ferromagnetic foreign body, which is an absolute contraindication to MRI. In such cases, low-dose non-contrast CT of the abdomen and pelvis is the recommended alternative. Low-dose CT is highly sensitive for urolithiasis, and the fetal radiation exposure (typically

<5 \text{ mGy}

) is well below the threshold for teratogenesis (

50 \text{ mGy}

Step-by-Step Solution

Evaluate the initial diagnostic presentation

The patient is pregnant and has symptoms of acute nephrolithiasis with an indeterminate renal ultrasound (physiologic hydronephrosis of pregnancy vs. true obstruction).

To establish the baseline clinical scenario and the need for further evaluation due to intractable pain.

Identify the standard secondary imaging modality

Magnetic resonance (MR) urography without contrast is typically the next preferred test in pregnancy.

MRI avoids ionizing radiation to the fetus while providing excellent visualization of the urinary tract.

Assess for patient-specific contraindications to the standard modality

The patient has a retained intraocular ferromagnetic foreign body.

This is an absolute contraindication to MRI, ruling out MR urography as an option.

Select the most appropriate alternative imaging study

Low-dose non-contrast CT of the abdomen and pelvis is selected.

It is highly sensitive for nephrolithiasis and exposes the fetus to less than

5 \text{ mGy}

of radiation, which is well below the teratogenic threshold of

50 \text{ mGy}

Key Concept

In pregnant patients with suspected nephrolithiasis where initial ultrasonography is indeterminate and MRI is contraindicated, a low-dose non-contrast CT of the abdomen and pelvis is the most appropriate next imaging study.

Estimated Time:1m 30s

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