USMLE STEP 1 BIOCHEMISTRY UPDATED QUESTIONS AND
ANSWERS
infant with somnolence, vomiting, cerebral edema, decreased BUN, hyperammonemia -
CORRECT ANSWER✅✅✅ornithine transcarbamoylase deficiency
infantile cataracts, doesn't track objects, absent social smile - CORRECT
ANSWER✅✅✅galactokinase deficiency
infantile cataracts, hepatomegaly, jaundice, failure to thrive - CORRECT
ANSWER✅✅✅uridyltransferase deficiency
causes of Down syndrome - CORRECT ANSWER✅✅✅1) meiotic nondisjunction (95%)
2) unbalanced robertsonian translocation (extra arm on Ch 21)
3) mosaic trisomy 21
treatment of hyperammonemia (HCV, urea cycle deficiency) - CORRECT
ANSWER✅✅✅limit protein in diet
benzoate, phenylbutarate (bind a.a.'s, increased aa exretion)
actulose (acidify GI tract, trap NH4 for excretion)
effect of hyperammonemia on metabolism - CORRECT ANSWER✅✅✅decreased alpha
ketoglutarate, inhibiting TCA cycle
Tay-Sach's (enzyme, accumulated substance) - CORRECT ANSWER✅✅✅hexosaminidase,
GM2 ganglioside
metachromatic leukodystrophy (enzyme, accumulated substance) - CORRECT
ANSWER✅✅✅arylsulfatase, cerebroside sulfate
, fabry's (enzyme, accumulated substance) - CORRECT ANSWER✅✅✅alpha-galactosidase,
ceramide trihexoside
krabbe's (enzyme, accumulated substance) - CORRECT ANSWER✅✅✅beta-
galactocerebrosidase, galactocerebroside
gauche's (enzyme, accumulated substance) - CORRECT
ANSWER✅✅✅glucocerebrosidase, glucocerebroside
niemann-pick - CORRECT ANSWER✅✅✅sphingomyelinase, sphingomyelin
hepatosplenomegaly, aseptic necrosis of femur, bone crises, macrophages that look like
crumpled tissue paper - CORRECT ANSWER✅✅✅Gaucher's sphingolipidosis
(glucocerebroside accumulation)
cherry macula, neurodegeneration, foam cells, hepatomegaly - CORRECT
ANSWER✅✅✅niemann-pick (sphyngomyelin)
cherry macula, neurodegenration, lysosomes with onion skin, NO hepatisplenomegaly -
CORRECT ANSWER✅✅✅tay-sachs (GM2 ganglioside)
causes of albinism - CORRECT ANSWER✅✅✅tyrosine transporter defect
tyrosine hydroxylase deficiency
lack of migration of neural crest cells (melanocytes)
where do preganglionic parasympathetic neurons originate? - CORRECT
ANSWER✅✅✅nuclei of cranial nerves
sacral spinal cord (S2-S4)
ANSWERS
infant with somnolence, vomiting, cerebral edema, decreased BUN, hyperammonemia -
CORRECT ANSWER✅✅✅ornithine transcarbamoylase deficiency
infantile cataracts, doesn't track objects, absent social smile - CORRECT
ANSWER✅✅✅galactokinase deficiency
infantile cataracts, hepatomegaly, jaundice, failure to thrive - CORRECT
ANSWER✅✅✅uridyltransferase deficiency
causes of Down syndrome - CORRECT ANSWER✅✅✅1) meiotic nondisjunction (95%)
2) unbalanced robertsonian translocation (extra arm on Ch 21)
3) mosaic trisomy 21
treatment of hyperammonemia (HCV, urea cycle deficiency) - CORRECT
ANSWER✅✅✅limit protein in diet
benzoate, phenylbutarate (bind a.a.'s, increased aa exretion)
actulose (acidify GI tract, trap NH4 for excretion)
effect of hyperammonemia on metabolism - CORRECT ANSWER✅✅✅decreased alpha
ketoglutarate, inhibiting TCA cycle
Tay-Sach's (enzyme, accumulated substance) - CORRECT ANSWER✅✅✅hexosaminidase,
GM2 ganglioside
metachromatic leukodystrophy (enzyme, accumulated substance) - CORRECT
ANSWER✅✅✅arylsulfatase, cerebroside sulfate
, fabry's (enzyme, accumulated substance) - CORRECT ANSWER✅✅✅alpha-galactosidase,
ceramide trihexoside
krabbe's (enzyme, accumulated substance) - CORRECT ANSWER✅✅✅beta-
galactocerebrosidase, galactocerebroside
gauche's (enzyme, accumulated substance) - CORRECT
ANSWER✅✅✅glucocerebrosidase, glucocerebroside
niemann-pick - CORRECT ANSWER✅✅✅sphingomyelinase, sphingomyelin
hepatosplenomegaly, aseptic necrosis of femur, bone crises, macrophages that look like
crumpled tissue paper - CORRECT ANSWER✅✅✅Gaucher's sphingolipidosis
(glucocerebroside accumulation)
cherry macula, neurodegeneration, foam cells, hepatomegaly - CORRECT
ANSWER✅✅✅niemann-pick (sphyngomyelin)
cherry macula, neurodegenration, lysosomes with onion skin, NO hepatisplenomegaly -
CORRECT ANSWER✅✅✅tay-sachs (GM2 ganglioside)
causes of albinism - CORRECT ANSWER✅✅✅tyrosine transporter defect
tyrosine hydroxylase deficiency
lack of migration of neural crest cells (melanocytes)
where do preganglionic parasympathetic neurons originate? - CORRECT
ANSWER✅✅✅nuclei of cranial nerves
sacral spinal cord (S2-S4)
