Vascular Anatomy, Human Anatomy, USMLE Step 1 - Full Vignette with Extended Explanations

A 21-year-old woman with known antiphospholipid antibody syndrome and a history of DVT presents with months of exertional dyspnea, chest discomfort, palpitations, and progressive fatigue. Family history suggests connective tissue disease, and exam findings include marfanoid body habitus and aortic dilation on imaging. How should clinicians select the optimal dimensional criterion for assessing surgical risk in a young adult with these complex features? Which clinical and imaging characteristics should be prioritized for informed management decisions?

VIDEO INFO
Category: Vascular Anatomy, Human Anatomy, USMLE Step 1
Difficulty: Hard - Advanced level - Challenges experienced practitioners
Question Type: Recent Changes
Case Type: Common Scenario

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QUESTION
A 21-year-old woman with antiphospholipid antibody syndrome (unprovoked left femoral DVT at age 19; warfarin-managed with steady INRs) presents after a brief opioid-associated hypoventilation episode that resolved with titrated intravenous naloxone. Over the past 8 months she notes exertional dyspnea, nonexertional chest pressure, intermittent palpitations, orthopnea requiring two pillows, and progressive fatigue without fevers or weight loss. She denies stimulant use and pregnancy....

OPTIONS
A. Apply the cross-sectional area/height index from gated CT: calculate area at the maximal ascending segment using inner-edge to inner-edge in end-diastole and refer if area (cm2) divided by height (m) is =10 cm2/m, even when absolute diameter is less than 5.5 cm.
B. Use a body-surface-area-indexed aortic size index =2.75 cm/m2 based on outer-edge to outer-edge CT measurements as the primary trigger for elective repair regardless of stature.
C. Adopt a height-adjusted absolute diameter rule and refer when the ascending diameter is =4.5 cm in adults less than 1.65 m tall, irrespective of cross-sectional area/height or valve phenotype.
D. Use the cross-sectional area/height index measured at the sinotubular junction on a non-ECG-gated CTA in systole with outer-edge calipers and refer when it reaches =10 cm2/m.

CORRECT ANSWER
A. Apply the cross-sectional area/height index from gated CT: calculate area at the maximal ascending segment using inner-edge to inner-edge in end-diastole and refer if area (cm2) divided by height (m) is =10 cm2/m, even when absolute diameter is less than 5.5 cm.

EXPLANATION
Basic teaching point: For thoracic aortic aneurysm risk stratification in smaller adults, contemporary guidelines prioritize height-indexed cross-sectional area measured on ECG-gated CT in end-diastole using inner-edge to inner-edge calipers. A threshold of 10 cm2/m identifies patients whose dissection risk is disproportionately high despite absolute diameters below classic 5.5 cm triggers. In this patient, the mid-ascending diameter is 4.8 cm; the cross-sectional area is x(2.4 cm)2 = 18.096 cm2. Dividing by her height of 1.58 m yields 11.45 cm2/m, exceeding the 10 cm2/m threshold and supporting early referral at a high-volume aortic center. This convention also standardizes phase (end-diastole) and edge (inner-to-inner) for CT, reducing motion and caliper bias.

Advanced teaching point: Close alternatives fail on either indexing construct or measurement convention. A body-surface-area-indexed aortic size index is not the primary adult trigger in current guidance and outer-edge calipers on CT overestimate size....


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