Metabolic, Nutritional and Immunologic Disorders
Core concepts
Know newborn screening principles and the disorders it targets: aminoacidopathies, fatty acid oxidation defects, organic acidemias, and endocrine disorders (congenital hypothyroidism, CAH). Understand that many inborn errors present with acute metabolic crisis-hypoglycemia, hyperammonemia, metabolic acidosis-and that the autopsy may be grossly normal, making biochemical and genetic testing essential.
Immunologic disorders include primary immunodeficiencies (SCID, chronic granulomatous disease, complement defects) and their infectious susceptibilities. Recognize nutritional deficiencies with pediatric significance (iron deficiency anemia, vitamin D deficiency/rickets, vitamin K deficiency bleeding) and the pathology of protein-energy malnutrition. Understand how maternal nutrition and placental transfer shape neonatal stores.
Traps
Examiners test the timing of presentation: urea cycle defects present after protein feeding with hyperammonemia but normal acid-base early; organic acidemias present with acidosis. Do not confuse vitamin K deficiency bleeding (classic, early, late forms) with hemophilia. Another trap: attributing sudden infant death to metabolic disease without evidence-consider it but do not overdiagnose.
Worked example / rule of thumb
A well-appearing newborn who deteriorates after feeding with vomiting, lethargy, and hyperammonemia: think urea cycle defect. Rule of thumb: "Fed → hyperammonemia (urea cycle); fasting → hypoglycemia + fatty acid oxidation defect."