Practice question

Cholesterol and phospholipid homeostasis in hepatocytes involves coordinated action of several ABC transporters at canalicular membrane. ABCB4 MDR3 flips phosphatidylcholine from inner to outer leaflet making it available for bile salt extraction forming mixed micelles that protect biliary epithelium and solubilize cholesterol exported by ABCG5/G8 heterodimer. ABCB1 MDR1 primarily exports hydrophobic xenobiotics and drugs, not bulk phospholipid for bile formation, though it can translocate short-chain lipids in vitro. ABCA1, not listed among options but relevant, mediates phospholipid and cholesterol efflux to apolipoprotein A1 for nascent HDL biogenesis at basolateral side. SGLT family members SLC5A1 and SLC5A2 are sodium-glucose symporters secondary active, unrelated to lipid transport. Mutations in ABCB4 cause progressive familial intrahepatic cholestasis type 3 with low biliary phospholipid, cholangitis and cholelithiasis, while ABCG5/G8 defects cause sitosterolemia, demonstrating specific partitioning of cholesterol and phosphatidylcholine pathways among distinct ABC transporters in liver physiology and disease. Such detailed mechanistic insight is frequently examined in competitive tests including NEET, CUET, CSIR-NET and GATE where transporter classification, energetics and disease linkage are integrated into problem-solving questions.