<prompt> Most important feature to diagnose fetal aneuploidy:Absent nasal boneNuchal translucencyIncrease in frontomaxillary angleReversal of ductus venosus flow  <response>: B i.e. Nuchal translucencyRef: Current Progress in Obstetrics and Gynecology; John Stud, Volume 1, page 17Explanation:Ultrasound Markers of Fetal Aneuploidy:Nuchal translucency is the single most powerful tool. More than 3 mm needs evaluation.Absence of nasal bone.Absence or reversal of flow in the ductus venosus on Doppler.Tricuspid regurgitation.Increase in frontomaxillary angle more than 85 deg. 