## Distinguishing MSUD from PKU ### Temporal and Clinical Presentation **Key Point:** MSUD presents acutely within the first 1–2 weeks of life with severe neurological symptoms (encephalopathy, seizures, hypertonia, lethargy), whereas PKU presents insidiously with gradual developmental delay over months if untreated. ### Comparison Table | Feature | MSUD | PKU | | --- | --- | --- | | **Enzyme defect** | Branched-chain α-ketoacid dehydrogenase | Phenylalanine hydroxylase | | **Elevated amino acids** | Leucine, isoleucine, valine | Phenylalanine | | **Age of onset** | 3–7 days (neonatal emergency) | 3–6 months (insidious) | | **Acute neurological signs** | Encephalopathy, seizures, hypertonia, lethargy | Absent in neonatal period | | **Urine odor** | Sweet, maple syrup-like | Musty, mousy | | **Progression if untreated** | Rapid (death in weeks) | Gradual (developmental delay) | | **Newborn screening** | Elevated BCAAs | Elevated phenylalanine | ### Why This Discriminates **High-Yield:** MSUD is a medical emergency in the neonatal period. Accumulation of branched-chain amino acids and their keto-metabolites (especially leucine) is directly neurotoxic, causing acute cerebral edema and metabolic acidosis. PKU, by contrast, does not cause acute neonatal encephalopathy; the neurological damage is chronic and preventable with early dietary restriction. **Clinical Pearl:** A neonate with acute encephalopathy, hypertonia, and sweet-smelling urine in the first 2 weeks of life is MSUD until proven otherwise. This requires immediate plasma amino acid analysis and urgent dietary intervention (restriction of BCAAs and supplementation with BCAA-free formula). ### Mnemonic **MAPLE = ACUTE:** **M**aple syrup urine disease = **A**cute neonatal emergency. **PKU = SLOW:** Phenylketonuria = **S**low, insidious neurological damage.
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