A clinical study is undertaken with subjects from families in which complications of atherosclerotic cardiovascular disease and tendinous xanthomas occurred before age 30 years. Some of the children in these families are observed to have early atheroma formation. These affected individuals benefit from treatment with pharmacologic agents that inhibit HMG-CoA reductase. Affected individuals in these families are most likely to have a mutation in a gene encoding a cell surface receptor for which of the following?