C

Free (un-esterified) cholesterol is the metabolically “active” pool that inserts into cell membranes, participates in signaling, and serves as substrate for steroid and bile-acid synthesis; esterified cholesterol is an inert storage/transport form carried in lipoproteins [Lange, 2020, PMID 32930466; Saito, 2024, PMID 38101585].