Short answers, pulled from the story.
Jaundice in adults is caused by abnormal bilirubin metabolism, liver dysfunction, or obstruction of the bile ducts. In the developed world, bile duct blockage (most often from gallstones) and medication-induced causes are the most common. In the developing world, infectious diseases such as viral hepatitis, leptospirosis, schistosomiasis, and malaria are the leading drivers.
More than 80 percent of newborns experience jaundice in their first week of life because their liver enzymes are immature, their gut microbiota is underdeveloped, and fetal hemoglobin breaks down rapidly after birth, all of which raises bilirubin levels. Most cases resolve without treatment.
Jaundice typically becomes visible when blood bilirubin levels exceed 2-3 mg/dl (34-51 micromoles per liter). Normal bilirubin levels are below 1.0 mg/dl. Yellowing of the sclera, the white of the eye, appears at a serum bilirubin of at least 3 mg/dl.
Both conditions yellow the skin, but carotenemia concentrates discoloration on the palms and soles and does not affect the sclera or the inside of the mouth. Jaundice produces diffuse yellowing across the body and typically involves the whites of the eyes. Carotenemia results from overconsumption of carotene-containing foods such as carrots, squash, and leafy vegetables.
When serum bilirubin exceeds 4-21 mg/dl in a newborn, treatment depends on the infant's age and prematurity. Phototherapy using a bili light is the standard first-line approach. Exchange transfusion is used in more severe cases. Frequent feedings also help by increasing bilirubin excretion through bowel movements and urination.
Kernicterus is permanent brain damage caused when unconjugated bilirubin crosses the blood-brain barrier in newborns with severe jaundice. Newborns are especially vulnerable because their blood-brain barrier is more permeable at high unconjugated bilirubin levels, and fetal hemoglobin breakdown adds to the bilirubin load. Cases of kernicterus have been rising in recent years, linked to reduced sunlight exposure in infants.