Beyond the Typical Marfan Phenotype: Severe Aortic Stenosis Caused by a Unicommissural Unicuspid Aortic Valve in Adolescence.

Unicuspid aortic valve (UAV) is an exceptionally rare congenital aortic valve anomaly. UAV is more commonly associated with significant aortic stenosis at a younger age than bicuspid aortic valve. Two forms of UAV have been described. Acommissural type of UAV typically presents in early infancy, and unicommissural type of UAV usually presents in late childhood or early adolescence. Echocardiography is the main imaging tool used to assess aortic valve morphology, aortic dimensions, and the presence and severity of stenosis or regurgitation. Early surgical intervention is typically required for UAV and is most commonly performed for aortic valve replacement. Postoperative echocardiograms are obtained on a regular basis to assess for proper function of the replaced valve and for signs of left ventricular dysfunction.  We present the case of a 16-year-old girl with genetically confirmed Marfan syndrome who presented with progressive exertional dyspnea, fatigue, and reduced exercise tolerance over several months and was found to have severe aortic stenosis due to unicommissural UAV. Computed tomography angiography demonstrated mid-ascending aortic dilation measuring 43 mm, corresponding to an indexed diameter of 25.0 mm/m² based on a body surface area of 1.72 m², while the aortic root measured 34 mm with a z-score of +1.8. She underwent successful surgical aortic valve replacement with a 19-mm mechanical prosthesis. Isolated aortic valve replacement was selected because the symptomatic severe valve obstruction required surgery, whereas the aortic root remained below the recommended surgical thresholds and the 43-mm mid-ascending aortic dilation was not considered an independent indication for replacement. No prior imaging was available to determine the aortic growth rate, and there was no confirmed family history of aortic dissection. The postoperative echocardiogram showed a well-functioning aortic prosthetic valve with preserved left ventricular systolic function. She was discharged on warfarin with structured international normalized ratio monitoring. Given her age, sex, significant aortopathy, mechanical prosthesis, and potential for future pregnancy, she received counseling regarding contraception, anticoagulation, pregnancy risk, and long-term aortic surveillance. Genetic counseling and cascade testing for the pathogenic fibrillin-1 gene variant were recommended for first-degree relatives. At the three-month follow-up, exertional dyspnea had improved, and she had resumed her usual daily activities.