What is the most likely cause of jaundice in a 4-day-old healthy child?

Most Likely Cause: Physiological Jaundice

In a 4-day-old healthy infant with jaundice and no other abnormal findings, physiological jaundice is the most likely diagnosis, representing the normal transitional hyperbilirubinemia that occurs in the majority of newborns. 1

Clinical Reasoning

Timing is Critical

• Day 4 of life falls within the classic peak period for physiological jaundice, which typically appears after 24 hours of age and peaks between days 3-5 in term infants 1, 2
• The timing essentially rules out pathological causes that present earlier (Rh hemolysis, ABO incompatibility, G6PD deficiency typically manifest within the first 24-48 hours with rapidly rising bilirubin) 1

Why Not the Other Options?

Rh Hemolysis (Option A):

• Would present earlier (within first 24-48 hours) with rapidly progressive jaundice requiring urgent treatment 1
• Typically accompanied by anemia, hepatosplenomegaly, and signs of hemolysis - none mentioned in this "otherwise unremarkable" infant 2

Breast Milk Jaundice (Option B):

• Breast milk jaundice specifically refers to prolonged unconjugated hyperbilirubinemia extending into the third week of life and beyond 3
• Day 4 is too early for this diagnosis - breast milk jaundice is a diagnosis of exclusion after the first 2 weeks 3
• This differs from "breastfeeding jaundice" (insufficient intake), which can occur earlier but would show signs of dehydration or poor feeding 4, 3

G6PD Deficiency (Option C):

• Would typically present with severe, rapidly rising jaundice in the first 2-3 days, often requiring phototherapy 1
• Usually triggered by oxidative stress and presents more dramatically than described 1

Other Pathological Causes (Option D):

• Sepsis, metabolic disorders, or biliary obstruction would present with additional clinical signs beyond isolated jaundice 1, 2
• The description "otherwise unremarkable" argues strongly against pathological etiologies 2

Key Clinical Distinction

The critical feature here is that the infant is "healthy" and "otherwise unremarkable." 1 Physiological jaundice is:

• A benign transitional event occurring in the majority of term infants 2
• Caused by normal breakdown of fetal red blood cells combined with immature hepatic conjugation 1
• Self-limited and typically resolves without treatment, though some infants may require brief phototherapy 1, 2

Management Approach

• Measure serum bilirubin level to determine if phototherapy is needed based on age-specific nomograms 5
• Ensure adequate feeding (8-12 times per day if breastfeeding) to promote bilirubin excretion 5
• If breastfeeding, assess for adequate intake - decreased feeding frequency (<8-9 times daily) is associated with higher bilirubin levels 5
• Follow-up bilirubin measurement within 24 hours if phototherapy is initiated and discontinued before day 3-4 5

Common Pitfall to Avoid

Do not confuse early breastfeeding-associated jaundice (insufficient intake) with breast milk jaundice (late-onset, prolonged). 4, 3 At day 4, if breastfeeding is involved and intake is adequate, this remains physiological jaundice. True breast milk jaundice is diagnosed after 2 weeks of age. 3

Answer: Physiological jaundice is the correct diagnosis - the timing, clinical presentation, and absence of other findings make this the only appropriate choice among the options provided.