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Difficulty: EasyPediatric Genetic and Metabolic Disorders

Match each classic pediatric clinical presentation with the corresponding defective enzyme.

  • A 2-week-old neonate presents with poor feeding, progressive lethargy, and diapers that smell distinctly sweet, like burnt sugar.Branched-chain alpha-keto acid dehydrogenase
  • A 4-year-old boy presents with intellectual disability, fair skin, blond hair, and a strong 'musty' body odor.Phenylalanine hydroxylase
  • A 9-year-old girl with a tall stature and long, thin fingers presents with sudden onset of hemiparesis and downward lens subluxation.Cystathionine beta-synthase
  • A 6-month-old infant is brought to the clinic because the parents noticed the infant's urine turns black when left standing in the diaper.Homogentisate oxidase

Answer

The neonate with sweet-smelling urine matches Branched-chain alpha-keto acid dehydrogenase. The boy with a musty odor matches Phenylalanine hydroxylase. The girl with a marfanoid habitus and thrombosis matches Cystathionine beta-synthase. The infant with dark urine matches Homogentisate oxidase.
Each pediatric metabolic disorder is matched to its corresponding enzymatic defect based on distinct, highly tested pathognomonic clinical signs: burnt sugar odor (MSUD), musty odor (PKU), marfanoid habitus with thrombosis (Homocystinuria), and dark urine (Alkaptonuria).

Step-by-Step Solution

1
Analyze the first clinical presentation (burnt sugar odor, lethargy).
Identify as Maple Syrup Urine Disease.
The distinctive odor of branched-chain amino acids in the urine strongly points to an inability to break down leucine, isoleucine, and valine.
2
Analyze the second presentation (musty odor, fair pigmentation, intellectual disability).
Identify as Phenylketonuria (PKU).
Accumulation of phenylalanine causes the musty odor, and decreased tyrosine (and subsequently melanin) synthesis leads to fair skin.
3
Analyze the third presentation (marfanoid body, thrombosis, downward lens subluxation).
Identify as Homocystinuria.
Thrombosis and downward lens subluxation differentiate this disorder from Marfan syndrome, pointing to an underlying amino acid metabolism defect.
4
Analyze the fourth presentation (urine darkening upon standing).
Identify as Alkaptonuria.
Homogentisic acid oxidizes in the air, causing the classic black discoloration of urine.

Key Concept

Clinical recognition and biochemical etiology of classic pediatric amino acid disorders.
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Topics

Pediatric Genetic and Metabolic Disorders

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