Biochemistry
High Yield USMLE Step 1 Notes
Welcome to the Biochemistry High-Yield Notes for Step 1.
These are the most commonly tested topics on the exam—ideal for quick review in your last
2 weeks before the exam, when you're short on time, or when you're just starting out and
want to focus on what is high yield. Created and peer-reviewed by physicians who scored
260+ on Step 1 in a Q&A style to reinforce active learning.
How to use this file
y
Read the question ➔ cover the answer ➔ self-quiz before revealing.
u
Review the NOTES ➔ they include high-yield details you don't want to
miss.
h G Need tutoring? Scan the QR code or visit The Match Guy Tutoring.
c
Watch the video explanations that go along with this file for deeper
t
understanding.
Ma
VITAMINS
e
1. A 6-year-old boy from a developing country presents with night blindness and frequent
respiratory infections. Eye exam shows conjunctival spots with a foamy appearance. What is
Th the most likely deficiency?
➔ Retinoic acid
Note: Bitot’s spots, xerophthalmia, and recurrent infections point to vitamin A deficiency.
Vitamin A is crucial for epithelial integrity and immune function.
2. A 29-year-old woman taking isotretinoin for acne reports nausea, dizziness, and hair
thinning. On fundoscopic exam, she shows signs of papilledema. What vitamin toxicity is the
likely cause?
➔ Vitamin A
Note: Hypervitaminosis A can cause intracranial hypertension(presented as papilledema
in this patient), alopecia, hepatotoxicity, and is teratogenic — always test for pregnancy
before treatment and OCPS are indicated
© 2025 TheMatchGuy LLC. All rights reserved. Page 1
Version 1.0 May 2025. No part of this document may be reproduced or distributed without written permission. For personal study only. Medical information
herein is for educational purposes and does not constitute clinical advice. Always confirm with current guidelines and your institution’s policies.
,3. A chronic alcoholic is brought in with confusion, ataxia, and ophthalmoplegia. His glucose
level is low, and he is given IV dextrose. His condition rapidly worsens. Diagnosis?
➔ Wernickes encephalopathy
Note: This patient with ataxia, confusion and ophthalmoplegia has wernickes
encephalopathy.
Always give thiamine before glucose in alcoholics to prevent Wernicke encephalopathy.
Thiamine is a cofactor for α-ketoglutarate Dehydrogenase in krebs cycle,
transketolase, and pyruvate DH
4. A malnourished patient with a history of alcohol use presents with memory loss and
confabulation. He does not have eye movement abnormalities or ataxia. What is the most
likely diagnosis?
➔ Korsakoff
y
Note: Korsakoff psychosis(causing retrograde amnesia with confabulations (making up
u
stories).
Results from chronic thiamine deficiency and mammillary body damage.
G
Irreversible and often follows untreated Wernicke encephalopathy.
tch
5. A 5-year-old child on a corn-based diet presents with symmetric skin lesions in sun-
exposed areas, diarrhea, and confusion. What vitamin is most likely deficient?
➔ Niacin
Ma
Note: Pellagra = 3 D’s: dermatitis (usually broad collar rash), diarrhea, dementia.
Also seen in Hartnup disease (⇓ tryptophan absorption), isoniazid use (⇓ B6-dependent
niacin synthesis) and carcinoid syndrome (consumption of tryptophan used in making
e
serotonin).
Th
6. A patient on high-dose niacin for hyperlipidemia complains of painful facial flushing after
each dose. What substance mediates this reaction?
➔ Prostaglandin
Note: Niacin-induced flushing is prostaglandin-mediated.
Pretreatment with aspirin reduces symptoms.
Niacin also increases uric acid (➞ gout risk ) and causes hyperglycemia.
7. A woman treated for TB develops numbness in her hands and feet. She also complains of
irritability. Which vitamin deficiency is most likely responsible?
➔ Pyridoxine B6
Note: TB patients are on RIPE therapy (Rifampin, Isoniazid, Pyrazinamide, Ethambutol)
Isoniazid inhibits vitamin B6 (pyridoxine), a cofactor in neurotransmitter synthesis and
transamination.
Deficiency ➞ peripheral neuropathy, seizures.
© 2025 TheMatchGuy LLC. All rights reserved. Page 2
,8. A child has dry skin, hair loss, and irritability. He consumes raw egg shakes daily as part of
a bodybuilding diet. What vitamin is most likely deficient?
➔ Biotin B7
Note: Avidin in raw egg whites binds biotin and prevents its absorption.
Biotin is a cofactor for carboxylases in gluconeogenesis (Pyruvate carboxylase) and FA
synthesis (acetyl Co-A carboxylase).
9. A 26-year-old pregnant woman presents late to prenatal care. Her baby is later born with a
neural tube defect. What vitamin deficiency is most likely responsible?
➔ Folate B9
Note: Folate (B9) is essential for DNA synthesis. Deficiency during early pregnancy can
lead to neural tube defects such as spina bifida and anencephaly.
Supplementation is recommended preconceptionally.
uy
10. A 70-year-old man presents with fatigue and glossitis. Labs show macrocytic anemia with
G
hypersegmented neutrophils. Neurologic exam is normal. Which vitamin is most likely
deficient?
➔ Folate
tch
Note: Folate deficiency caused by poor diet (stores deplete in weeks), alcoholism,
pregnancy, drugs like methotrexate and phenytoin.
a
Folate deficiency presents with Megaloblastic anemia with no neurological signs.
Both folate and vitamin B12 deficiencies ➞ megaloblastic anemia + ⇑ homocysteine
e M
11. A 60-year-old woman presents with numbness in her legs, difficulty walking, and loss of
h
vibration sense. She also has macrocytic anemia and elevated methylmalonic acid levels.
What is the most likely cause?
T
➔ Cobalamin
Note: Vitamin B12 (cobalamin) deficiency causes subacute combined degeneration➞
demyelination of corticospinal tracts( weakness and UMNL symptoms), dorsal
columns(ataxia, loss proprioception ), and spinocerebellar tracts.
B12 deficiency ➞ ⇑ methylmalonic acid + neurologic symptoms (e.g., paresthesia,
ataxia)
B12 deficiency: develops over years, especially in vegans without supplementation
(Folate is found in leafy greens, not typically deficient in vegan diets)
12. A 52-year-old woman presents with fatigue, paresthesias, and glossitis. Laboratory tests
reveal macrocytic anemia. Further workup shows anti–intrinsic factor antibodies. Which of
the following deficiencies is most likely contributing to her symptoms?
➔ Cobalamin
Note: Intrinsic factor is required for B12 absorption in the terminal ileum.
Pernicious anemia presents with antibodies against intrinsic factor ➞ B12 absorption,
megaloblastic anemia and paraesthesia.
© 2025 TheMatchGuy LLC. All rights reserved. Page 3
, Other B12 deficiency causes include: Crohn’s disease(terminal ileitis and
resections), vegan diet, and gastric bypass.
13. A patient with vitamin B12 deficiency is treated with folate only. What complication may
worsen as a result?
➔ Neuropathy
Note: Giving folate alone may improve the anemia but consumes B12 and worsen the
neurologic damage in B12 deficiency. Always check both before treating.
14. A 9-month-old exclusively breastfed infant is brought in with bowed legs and delayed
closure of fontanelles. X-ray shows metaphyseal cupping and fraying. What vitamin
deficiency is most likely responsible?
➔ Vitamin D
uy
Note: Rickets is due to vitamin D deficiency in children.
Breast milk lacks adequate vitamin D, so supplementation is recommended—especially
in infants with limited sun exposure.
G
Fat malabsorption leads to loss of fat soluble vitamins including vitamin D.
Vitamin D deficiency can present with bony pains in adults.
tch
15. A middle-aged man with sarcoidosis develops hypercalcemia and kidney stones. His
a
PTH is low, but 1,25-dihydroxyvitamin D is elevated. What is the cause of his hypercalcemia?
➔ Macrophages
M
Note: In granulomatous diseases, macrophages express 1-α hydroxylase, leading to
unregulated vitamin D activation and hypercalcemia. This bypasses normal PTH
e
regulation.
h
Steroid treatment decreases the 1-α hydroxylase.
T
16. A premature infant develops necrotizing enterocolitis and is treated with prolonged
parenteral nutrition. He later develops hemolytic anemia and decreased deep tendon
reflexes. What vitamin deficiency is most likely?
➔ Vitamin E
Note: Vitamin E protects RBCs and neurons from oxidative damage. (vitamins A,C,E are
antioxidants that clear oxygen radicals).
Fat malabsorption in cystic fibrosis can cause Vitamin E deficiency (also vitamin
A,K,D).
Vitamin E deficiency presents with hemolytic anemia and posterior column
demyelination (mimics B12 deficiency but no methylmalonic acid elevation and
hemolytic anemia instead of megaloblastic anemia).
17. A patient with fat malabsorption due to cystic fibrosis develops prolonged PT and easy
bruising. He is given warfarin and experiences excessive bleeding. Which vitamin deficiency
most likely contributed?
➔ Vitamin K
© 2025 TheMatchGuy LLC. All rights reserved. Page 4
High Yield USMLE Step 1 Notes
Welcome to the Biochemistry High-Yield Notes for Step 1.
These are the most commonly tested topics on the exam—ideal for quick review in your last
2 weeks before the exam, when you're short on time, or when you're just starting out and
want to focus on what is high yield. Created and peer-reviewed by physicians who scored
260+ on Step 1 in a Q&A style to reinforce active learning.
How to use this file
y
Read the question ➔ cover the answer ➔ self-quiz before revealing.
u
Review the NOTES ➔ they include high-yield details you don't want to
miss.
h G Need tutoring? Scan the QR code or visit The Match Guy Tutoring.
c
Watch the video explanations that go along with this file for deeper
t
understanding.
Ma
VITAMINS
e
1. A 6-year-old boy from a developing country presents with night blindness and frequent
respiratory infections. Eye exam shows conjunctival spots with a foamy appearance. What is
Th the most likely deficiency?
➔ Retinoic acid
Note: Bitot’s spots, xerophthalmia, and recurrent infections point to vitamin A deficiency.
Vitamin A is crucial for epithelial integrity and immune function.
2. A 29-year-old woman taking isotretinoin for acne reports nausea, dizziness, and hair
thinning. On fundoscopic exam, she shows signs of papilledema. What vitamin toxicity is the
likely cause?
➔ Vitamin A
Note: Hypervitaminosis A can cause intracranial hypertension(presented as papilledema
in this patient), alopecia, hepatotoxicity, and is teratogenic — always test for pregnancy
before treatment and OCPS are indicated
© 2025 TheMatchGuy LLC. All rights reserved. Page 1
Version 1.0 May 2025. No part of this document may be reproduced or distributed without written permission. For personal study only. Medical information
herein is for educational purposes and does not constitute clinical advice. Always confirm with current guidelines and your institution’s policies.
,3. A chronic alcoholic is brought in with confusion, ataxia, and ophthalmoplegia. His glucose
level is low, and he is given IV dextrose. His condition rapidly worsens. Diagnosis?
➔ Wernickes encephalopathy
Note: This patient with ataxia, confusion and ophthalmoplegia has wernickes
encephalopathy.
Always give thiamine before glucose in alcoholics to prevent Wernicke encephalopathy.
Thiamine is a cofactor for α-ketoglutarate Dehydrogenase in krebs cycle,
transketolase, and pyruvate DH
4. A malnourished patient with a history of alcohol use presents with memory loss and
confabulation. He does not have eye movement abnormalities or ataxia. What is the most
likely diagnosis?
➔ Korsakoff
y
Note: Korsakoff psychosis(causing retrograde amnesia with confabulations (making up
u
stories).
Results from chronic thiamine deficiency and mammillary body damage.
G
Irreversible and often follows untreated Wernicke encephalopathy.
tch
5. A 5-year-old child on a corn-based diet presents with symmetric skin lesions in sun-
exposed areas, diarrhea, and confusion. What vitamin is most likely deficient?
➔ Niacin
Ma
Note: Pellagra = 3 D’s: dermatitis (usually broad collar rash), diarrhea, dementia.
Also seen in Hartnup disease (⇓ tryptophan absorption), isoniazid use (⇓ B6-dependent
niacin synthesis) and carcinoid syndrome (consumption of tryptophan used in making
e
serotonin).
Th
6. A patient on high-dose niacin for hyperlipidemia complains of painful facial flushing after
each dose. What substance mediates this reaction?
➔ Prostaglandin
Note: Niacin-induced flushing is prostaglandin-mediated.
Pretreatment with aspirin reduces symptoms.
Niacin also increases uric acid (➞ gout risk ) and causes hyperglycemia.
7. A woman treated for TB develops numbness in her hands and feet. She also complains of
irritability. Which vitamin deficiency is most likely responsible?
➔ Pyridoxine B6
Note: TB patients are on RIPE therapy (Rifampin, Isoniazid, Pyrazinamide, Ethambutol)
Isoniazid inhibits vitamin B6 (pyridoxine), a cofactor in neurotransmitter synthesis and
transamination.
Deficiency ➞ peripheral neuropathy, seizures.
© 2025 TheMatchGuy LLC. All rights reserved. Page 2
,8. A child has dry skin, hair loss, and irritability. He consumes raw egg shakes daily as part of
a bodybuilding diet. What vitamin is most likely deficient?
➔ Biotin B7
Note: Avidin in raw egg whites binds biotin and prevents its absorption.
Biotin is a cofactor for carboxylases in gluconeogenesis (Pyruvate carboxylase) and FA
synthesis (acetyl Co-A carboxylase).
9. A 26-year-old pregnant woman presents late to prenatal care. Her baby is later born with a
neural tube defect. What vitamin deficiency is most likely responsible?
➔ Folate B9
Note: Folate (B9) is essential for DNA synthesis. Deficiency during early pregnancy can
lead to neural tube defects such as spina bifida and anencephaly.
Supplementation is recommended preconceptionally.
uy
10. A 70-year-old man presents with fatigue and glossitis. Labs show macrocytic anemia with
G
hypersegmented neutrophils. Neurologic exam is normal. Which vitamin is most likely
deficient?
➔ Folate
tch
Note: Folate deficiency caused by poor diet (stores deplete in weeks), alcoholism,
pregnancy, drugs like methotrexate and phenytoin.
a
Folate deficiency presents with Megaloblastic anemia with no neurological signs.
Both folate and vitamin B12 deficiencies ➞ megaloblastic anemia + ⇑ homocysteine
e M
11. A 60-year-old woman presents with numbness in her legs, difficulty walking, and loss of
h
vibration sense. She also has macrocytic anemia and elevated methylmalonic acid levels.
What is the most likely cause?
T
➔ Cobalamin
Note: Vitamin B12 (cobalamin) deficiency causes subacute combined degeneration➞
demyelination of corticospinal tracts( weakness and UMNL symptoms), dorsal
columns(ataxia, loss proprioception ), and spinocerebellar tracts.
B12 deficiency ➞ ⇑ methylmalonic acid + neurologic symptoms (e.g., paresthesia,
ataxia)
B12 deficiency: develops over years, especially in vegans without supplementation
(Folate is found in leafy greens, not typically deficient in vegan diets)
12. A 52-year-old woman presents with fatigue, paresthesias, and glossitis. Laboratory tests
reveal macrocytic anemia. Further workup shows anti–intrinsic factor antibodies. Which of
the following deficiencies is most likely contributing to her symptoms?
➔ Cobalamin
Note: Intrinsic factor is required for B12 absorption in the terminal ileum.
Pernicious anemia presents with antibodies against intrinsic factor ➞ B12 absorption,
megaloblastic anemia and paraesthesia.
© 2025 TheMatchGuy LLC. All rights reserved. Page 3
, Other B12 deficiency causes include: Crohn’s disease(terminal ileitis and
resections), vegan diet, and gastric bypass.
13. A patient with vitamin B12 deficiency is treated with folate only. What complication may
worsen as a result?
➔ Neuropathy
Note: Giving folate alone may improve the anemia but consumes B12 and worsen the
neurologic damage in B12 deficiency. Always check both before treating.
14. A 9-month-old exclusively breastfed infant is brought in with bowed legs and delayed
closure of fontanelles. X-ray shows metaphyseal cupping and fraying. What vitamin
deficiency is most likely responsible?
➔ Vitamin D
uy
Note: Rickets is due to vitamin D deficiency in children.
Breast milk lacks adequate vitamin D, so supplementation is recommended—especially
in infants with limited sun exposure.
G
Fat malabsorption leads to loss of fat soluble vitamins including vitamin D.
Vitamin D deficiency can present with bony pains in adults.
tch
15. A middle-aged man with sarcoidosis develops hypercalcemia and kidney stones. His
a
PTH is low, but 1,25-dihydroxyvitamin D is elevated. What is the cause of his hypercalcemia?
➔ Macrophages
M
Note: In granulomatous diseases, macrophages express 1-α hydroxylase, leading to
unregulated vitamin D activation and hypercalcemia. This bypasses normal PTH
e
regulation.
h
Steroid treatment decreases the 1-α hydroxylase.
T
16. A premature infant develops necrotizing enterocolitis and is treated with prolonged
parenteral nutrition. He later develops hemolytic anemia and decreased deep tendon
reflexes. What vitamin deficiency is most likely?
➔ Vitamin E
Note: Vitamin E protects RBCs and neurons from oxidative damage. (vitamins A,C,E are
antioxidants that clear oxygen radicals).
Fat malabsorption in cystic fibrosis can cause Vitamin E deficiency (also vitamin
A,K,D).
Vitamin E deficiency presents with hemolytic anemia and posterior column
demyelination (mimics B12 deficiency but no methylmalonic acid elevation and
hemolytic anemia instead of megaloblastic anemia).
17. A patient with fat malabsorption due to cystic fibrosis develops prolonged PT and easy
bruising. He is given warfarin and experiences excessive bleeding. Which vitamin deficiency
most likely contributed?
➔ Vitamin K
© 2025 TheMatchGuy LLC. All rights reserved. Page 4
