A 24-yeаr-оld mаn is referred tо yоur prаctice for elevated cholesterol level. The patient has a father and two siblings with high cholesterol levels on medications. His 31-year-old brother recently had a myocardial infarction. On physical examination, the patient has bilateral arcus senilis. His extremities are remarkable for diffuse and nodular thickenings of the Achilles tendon. What is the most likely diagnosis?