Practice question

Hepatic bile secretion relies on coordinated action of several canalicular ABC transporters in hepatocytes. ABCB11 BSEP exports bile acids, ABCG5/ABCG8 heterodimer exports cholesterol, and ABCB4 previously called MDR2 in mice and MDR3 in humans is dedicated to phosphatidylcholine. ABCB4 acts as ATP-dependent floppase moving phosphatidylcholine from inner cytoplasmic leaflet to outer exoplasmic leaflet of canalicular membrane where bile salt micelles extract it into biliary space forming mixed micelles. This phospholipid shield protects cholangiocyte membranes from detergent attack by hydrophobic bile salts and solubilizes cholesterol preventing crystal formation. Mice lacking Abcb4 develop spontaneous cholangitis, bile duct proliferation, and biliary fibrosis due to regurgitation of low-phospholipid toxic bile. In humans, biallelic loss causes progressive familial intrahepatic cholestasis type 3 presenting in childhood, while heterozygous variants predispose to low-phospholipid-associated cholelithiasis, intrahepatic cholestasis of pregnancy and drug-induced cholestasis, demonstrating essential protective role in hepatic physiology. 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.