Pathophysiology and Natural History of Metabolic Disorders
Mechanistic categories
The candidate should be able to explain disease at the level of the pathway: intoxication disorders (urea cycle defects, organic acidemias, aminoacidopathies) accumulate toxic intermediates and often present after a symptom-free interval; energy deficiency disorders (fatty acid oxidation defects, mitochondrial disorders, glycogen storage diseases) present with hypoketotic hypoglycemia, lactic acidosis, or exercise intolerance; storage disorders (lysosomal and peroxisomal diseases) accumulate substrate and present with progressive organomegaly, neurodegeneration, or skeletal disease; and transport or signaling defects present with specific organ phenotypes.
Natural history and genotype–phenotype
Understand that many IEMs have a spectrum from severe neonatal presentation to attenuated adult-onset disease, and that the same enzyme deficiency can produce different phenotypes depending on residual activity, tissue expression, and environmental triggers (fasting, illness, protein load). Anticipatory guidance depends on knowing the expected trajectory: which patients are at risk for acute decompensation, which develop progressive organ damage, and which remain stable.
Traps
Questions often present a well child who decompensates with fasting or intercurrent illness and expect recognition of an energy-deficiency or intoxication disorder. Another trap is assuming a severe genotype always predicts a severe phenotype; variable expressivity and modifier effects are common.
Rule of thumb
Ask two questions for every disorder: What accumulates, and what is starved? The answer predicts the biochemical signature (e.g., elevated ammonia and low citrulline vs. hypoketotic hypoglycemia and elevated dicarboxylic acids) and the clinical triggers to counsel the family about.