March Week 1: RTA
Case: Why does my baby not grow?
A 4 months old infant presents to you in clinic. His weight and length are below the 3rd centile. At birth he had been full term without complication with a length and height on the 75th centile line. At 1 month he was on the 50th centile for length and weight. The baby appears well hydrated with a normal physical exam. He has 5 wet diapers per day, and stools twice daily. His diet consists of protein-based formula fortified to 22 kcal/oz. He feeds well and gets about 110 kcal/kg/day consistently.
In your despair you order some labs. The CBC is normal, as are the LFTs. The chemistry shows:
Na 151 meq/L
K 3 meq/L
Cl 125 meq/L
Bicarbonate 15 meq/L
Anion gap 11
BUN 13 mg/dL
Crea 0.4 mg/dL
You realize that the baby suffers from a hypernatremic, hyperchloremic metabolic acidosis with normal anion gap.
You wonder what is going on and ask yourself:
1. Where does the acidosis come from?
2. What do I need to do to figure it out?