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Metabolic reasoning
Whole-map thinking. Each case is a chain of cause → effect that ends in the labs: why DKA has ketones and hyperglycemia, why a drinker goes hypoglycemic, why an infant with MCAD deficiency spills no ketones. Walk through the chain on the map, then order it yourself, answer the why questions, or diagnose from the labs.
An infant with a big heart and no ketones
A 10-month-old presents with hypotonia, hepatomegaly and an episode of lethargy after a missed feed. Echocardiogram shows a dilated cardiomyopathy.
| Glucose | 40 mg/dL | ↓ |
| Urine ketones | negative | ↓ (inappropriate) |
| Plasma free carnitine | very low | ↓↓ |
| Acylcarnitines | low overall | ↓ |
| CK | ↑ | ↑ |
Primary carnitine deficiency (OCTN2 transporter)
No carnitine, no shuttle: long-chain fatty acids can't reach the matrix in any tissue — including the heart.
Condition card: Primary (systemic) carnitine deficiency →Pathway map →
- OCTN2 is the plasma-membrane carnitine transporter; without it cells can't take up carnitine and the kidney wastes it → very low plasma carnitine.
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Pathways involved: Lipolysis, carnitine shuttle & β-oxidation, Ketogenesis & ketone body use, Gluconeogenesis. Drill them one at a time in Metabolic pathways.