Down syndrome, or trisomy 21, has a characteristic constellation of clinical findings, including various congenital heart defects. We report a case of an adult male with Down syndrome who presented with a 3-week history of lower limb pain and swelling, attributed to cellulitis. Clinical and angiographic evaluation identified a below-knee mycotic pseudoaneurysm secondary to infective endocarditis. Surgical aneurysmal repair and revascularization were performed. Various management options are outlined in this report.