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Jaundice (icterus) = yellowish discoloration of skin, sclera, and mucous membranes due to hyperbilirubinemia (>2–2.5 mg/dL serum bilirubin).

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A man before and after a liver transplant.

Top comment: Girl, for reference

Physiology

Bilirubin Metabolism

Hemoglobin breakdown → unconjugated bilirubin (indirect, water-insoluble, albumin-bound).
Transported to liver → conjugated with glucuronic acid (by UGT enzyme) → conjugated bilirubin (direct, water-soluble).
Excreted in bile → intestine → converted by bacteria to urobilinogen & stercobilin (gives stool color).
A small fraction reabsorbed → excreted in urine as urobilinogen.

Classification

Type	Desciption	Causes
Pre-hepatic
(hemolytic)	Excess unconjugated bilirubin due to increased RBC breakdown.	• Hemolytic anemias
• Malaria
• Thalassemia
• Transfusion reaction	• ↑ Unconjugated bilirubin.
• Normal-colored stool, dark urine absent (since unconjugated not water-soluble).
Hepatic (hepatocellular)	Impaired uptake, conjugation, or secretion of bilirubin.	• Hepatitis (viral, alcoholic, autoimmune)
• Cirrhosis
• Drugs
• Gilbert’s/Crigler-Najjar	Mixed ↑ unconjugated + conjugated bilirubin; raised liver enzymes (AST, ALT).
Post-hepatic (obstructive / cholestatic)	Blockage of bile flow (intrahepatic or extrahepatic).	• Gallstones
• Cholangiocarcinoma
• Pancreatic head carcinoma
• Biliary atresia	• ↑ Conjugated bilirubin.
• Pale/clay-colored stools (lack stercobilin).
• Dark urine (bilirubinuria).
• Raised ALP, GGT.