Practice question

Phosphatidylcholine supply to bile is governed by canalicular ABC transporter ABCB4, also known as multidrug resistance protein 2 or 3 in rodents and humans. This ATP-driven floppase actively translocates endogenous phosphatidylcholine from inner cytoplasmic leaflet to outer exoplasmic leaflet of hepatocyte canalicular membrane. Bile salts secreted via ABCB11 then extract phospholipid into bile forming mixed micelles that solubilize cholesterol and shield ductal epithelium from detergent toxicity of bile acids. ABCB1 MDR1 contributes to hydrophobic xenobiotic efflux but does not move bulk phosphatidylcholine for bile; Na+/K+ ATPase generates sodium gradient but does not directly handle biliary lipids; GLUT family facilitators transport monosaccharides, not phospholipids. Mutations in ABCB4 cause progressive familial intrahepatic cholestasis type 3 and low-phospholipid-associated cholelithiasis, linking transporter dysfunction to gallstone disease, fibrosis and cholestasis, illustrating specific regulation of biliary lipid composition by dedicated floppase rather than generic drug exporter. 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.