**Core Concept**
Neonatal jaundice is a common condition in newborns characterized by elevated bilirubin levels, which can be caused by various factors, including hemolysis, liver dysfunction, and biliary obstruction. The timing and presentation of neonatal jaundice can provide clues to its underlying cause.

**Why the Correct Answer is Right**
Rh incompatibility typically presents with jaundice within the first 24-48 hours of life due to hemolysis of Rh-positive red blood cells, which is not consistent with the timing of 2nd week jaundice. In contrast, conditions like galactosemia, hypothyroidism, and breast milk jaundice typically present later in the first or second week of life.

**Why Each Wrong Option is Incorrect**
**Option A:** Galactosemia is a genetic disorder that impairs the body's ability to metabolize galactose, leading to elevated bilirubin levels and jaundice, which typically appears in the first week of life, not the second week.
**Option C:** Hypothyroidism can cause liver dysfunction and elevated bilirubin levels, but it usually presents later in the first month of life, not within the second week.
**Option D:** Breast milk jaundice is a condition where bilirubin levels rise due to substances in breast milk that increase bilirubin reabsorption, typically presenting in the second week of life, making it a plausible cause of 2nd week jaundice.

**Clinical Pearl / High-Yield Fact**
Hemolytic diseases of the newborn, such as Rh incompatibility, typically cause jaundice within the first 48 hours of life, whereas conditions like galactosemia and breast milk jaundice present later in the first or second week.

✓ Correct Answer: B. Rh Incompatibility