If you’re preparing for the United States Medical Licensing Examination® (USMLE®) Step 3 exam, you might want to know which questions are most often missed by test-prep takers. Check out this example from Kaplan Medical, and read an expert explanation of the answer. Also check out all posts in this series.
This month’s stumper
A 45-year-old man is brought to the emergency department after being struck by a motor vehicle. The patient is intoxicated and combative, but has a Glasgow Coma Scale score of 15. Primary survey reveals a well-developed man in mild distress. He is in a cervical collar. Vital signs are temperature 37.1°C (98.8°F), pulse 110/minute, respirations 26/minute and blood pressure 150/90 mm Hg.
He has obvious lower extremity tibia fractures bilaterally and a laceration on his forehead. Secondary survey with pelvic x-ray shows severe pelvic trauma with bilateral inferior and superior pubic ramus fractures. Focused abdominal ultrasonography for trauma (FAST) shows no free intraperitoneal fluid.
Which of the following is the most appropriate next step?
A. Abdominal CT scan.
B. Complete abdominal ultrasound.
C. Diagnostic peritoneal lavage.
D. Exploratory laparotomy.
E. Pelvic angiography.
The correct answer is A.
Kaplan Medical explains why
This patient has severe trauma involving his lower extremities and pelvis. The concern is for intra-abdominal trauma resulting in bleeding, despite the negative FAST exam. An abdominal CT scan is the imaging modality of choice, since it is more sensitive than ultrasound and detects both intraperitoneal and retroperitoneal blood.
Why the other answers are wrong
Choice B: A complete abdominal ultrasound is unlikely to yield additional information in the setting of a FAST exam that is negative for free fluid.
Choice C: Diagnostic peritoneal lavage is useful for the detection of free intraperitoneal blood in the setting of an equivocal FAST exam in a patient too unstable for a CT scan. This patient has a negative FAST exam and is not too unstable for a CT, and there is additional concern for retroperitoneal bleeding, given the pelvic fractures on X-ray.
Choice D: Exploratory laparotomy may be indicated depending on the CT scan findings, but in the setting of blunt trauma with a normal blood pressure and negative FAST exam, this would be incorrect. In a hemodynamically unstable patient, with a positive FAST exam, this would be the correct choice.
Choice E: A pelvic angiogram may become indicated to assess for pelvic bleeding that can be managed with endovascular embolization, but the first test to assess the injuries is a CT scan.
Tips to remember
- Blunt trauma should be evaluated with a primary and then secondary survey.
- Ancillary tests, including CT scan, should be performed in the hemodynamically stable patient only.
- FAST exam is useful for evaluating for intraperitoneal fluid, but cannot be used to assess the retroperitoneal space.
For more prep questions on USMLE Steps 1, 2 and 3, view other posts in this series.
