Dietary, cofactor and emergency management of metabolic disease

You are reviewing an 8-year-old boy with phenylketonuria who has been lost to metabolic follow-up. His recent blood phenylalanine is markedly above target, his growth has fallen two centiles, and his school reports worsening attention. His family have stopped the phenylalanine-free medical food. **Discuss:** (1) the principles of the dietary pillar in PKU, what a complete restricted diet requires, and the consequences of chronic non-adherence; (2) how you would assess and reintroduce therapy, including the role of BH4 (sapropterin) responsiveness testing; (3) the place of cofactor therapy across inborn errors of metabolism, with named examples and doses; (4) how the principles of the sick-day regimen apply to a different disorder — a child with a fatty-acid oxidation defect presenting after fasting.