A 38-year-old woman is found to have markedly elevated fasting triglycerides (520 mg/dL) and total cholesterol (380 mg/dL) on routine screening. LDL-C is 90 mg/dL and HDL-C is 28 mg/dL. She reports a family history of premature myocardial infarction in her father (age 48) and maternal uncle (age 52). She is not on any medications. Abdominal examination reveals hepatosplenomegaly. What is the most appropriate next step in management?