**Core Concept**
The scenario describes a newborn baby with signs and symptoms suggestive of hemolytic disease of the newborn (HDN), a condition where maternal antibodies against fetal red blood cells lead to hemolysis and jaundice. The Coombs test is positive, indicating the presence of these antibodies.

**Why the Correct Answer is Right**
The key to this question is understanding the progression of the disease. Initially, the baby has a high level of unconjugated bilirubin (TSB 5 mg/dL), which is a common finding in newborns with HDN. However, the concern here is the decrease in body temperature (97.7°F to 95°F) and the significant weight loss (60g) over 5 days. This indicates a severe infection, likely sepsis, which is a life-threatening complication in neonates. The presence of yellow, seedy stools (diarrhea) further supports this diagnosis.

**Why Each Wrong Option is Incorrect**
**Option A:** While jaundice is a common finding in HDN, the change in the parameter of most concern is not the increase in TSB, as it is expected in this condition.

**Option B:** The hemoglobin level has actually increased, which is not a typical finding in HDN.

**Option C:** While weight loss is a concern, it is not the most alarming change in this scenario, as it is often seen in newborns with HDN.

**Option D:** The change in body temperature is the most concerning parameter, as it indicates a severe infection, likely sepsis, which requires immediate attention.

**Clinical Pearl / High-Yield Fact**
In neonates, a temperature below 97.7°F (36.5°C) is a red flag for sepsis, and a weight loss of >10% of birth weight over the first 7 days of life is indicative of a severe infection.

**Correct Answer:** D. Decrease in body temperature.