USMLE STEP 1 – FULL PRACTICE EXAM (100
QUESTIONS) LATEST UPDATED 2026 2027 REAL
EXAM WITH ACCURATE QUESTIONS AND CORRECT
ANSWERS &RATIONALES {100% VERIFIED
ANSWERS} RATED A+
1. A 6-year-old boy presents with recurrent infections, eczema, and
thrombocytopenia. Which immunologic abnormality is most likely present?
A. Absent thymus
B. Defective CD40 ligand
C. Decreased IgA
D. Impaired actin polymerization
Answer: D
Rationale: Wiskott-Aldrich syndrome is caused by defective actin polymerization, leading
to immunodeficiency, eczema, and thrombocytopenia.
2. A patient has hypercalcemia, kidney stones, bone pain, and psychiatric
symptoms. Which hormone is most likely elevated?
A. Calcitonin
B. Vitamin D
C. Parathyroid hormone
D. Aldosterone
Answer: C
Rationale: Primary hyperparathyroidism causes elevated PTH, leading to hypercalcemia
and classic “stones, bones, groans, and psychiatric overtones.”
,3. Which enzyme is deficient in patients with classic phenylketonuria?
A. Tyrosinase
B. Phenylalanine hydroxylase
C. Homogentisate oxidase
D. Dihydrofolate reductase
Answer: B
Rationale: PKU results from deficiency of phenylalanine hydroxylase, causing
accumulation of phenylalanine and neurologic damage.
4. A patient with chronic alcoholism develops confusion, ophthalmoplegia, and
ataxia. Which brain structure is most affected?
A. Caudate nucleus
B. Mammillary bodies
C. Substantia nigra
D. Red nucleus
Answer: B
Rationale: Wernicke encephalopathy affects mammillary bodies due to thiamine
deficiency.
5. A drug decreases preload and causes venodilation. Which class does it belong
to?
A. Beta blockers
B. ACE inhibitors
C. Nitrates
D. Calcium channel blockers
Answer: C
Rationale: Nitrates primarily cause venodilation, reducing preload and myocardial oxygen
demand.
6. A newborn presents with ambiguous genitalia and salt-wasting crisis. Which
enzyme is most likely deficient?
, A. 21-hydroxylase
B. 17α-hydroxylase
C. 11β-hydroxylase
D. Aromatase
Answer: A
Rationale: 21-hydroxylase deficiency causes congenital adrenal hyperplasia with salt
wasting and virilization.
7. Which virus is most likely to cause erythema infectiosum?
A. Parvovirus B19
B. HHV-6
C. Measles virus
D. Rubella virus
Answer: A
Rationale: Parvovirus B19 causes fifth disease characterized by slapped-cheek rash.
8. A patient has difficulty abducting the arm beyond 15 degrees. Which muscle is
most likely injured?
A. Supraspinatus
B. Deltoid
C. Infraspinatus
D. Teres major
Answer: B
Rationale: The deltoid muscle is responsible for arm abduction beyond the initial 15
degrees.
9. Which neurotransmitter is decreased in Parkinson disease?
A. GABA
B. Acetylcholine
C. Dopamine
D. Serotonin
QUESTIONS) LATEST UPDATED 2026 2027 REAL
EXAM WITH ACCURATE QUESTIONS AND CORRECT
ANSWERS &RATIONALES {100% VERIFIED
ANSWERS} RATED A+
1. A 6-year-old boy presents with recurrent infections, eczema, and
thrombocytopenia. Which immunologic abnormality is most likely present?
A. Absent thymus
B. Defective CD40 ligand
C. Decreased IgA
D. Impaired actin polymerization
Answer: D
Rationale: Wiskott-Aldrich syndrome is caused by defective actin polymerization, leading
to immunodeficiency, eczema, and thrombocytopenia.
2. A patient has hypercalcemia, kidney stones, bone pain, and psychiatric
symptoms. Which hormone is most likely elevated?
A. Calcitonin
B. Vitamin D
C. Parathyroid hormone
D. Aldosterone
Answer: C
Rationale: Primary hyperparathyroidism causes elevated PTH, leading to hypercalcemia
and classic “stones, bones, groans, and psychiatric overtones.”
,3. Which enzyme is deficient in patients with classic phenylketonuria?
A. Tyrosinase
B. Phenylalanine hydroxylase
C. Homogentisate oxidase
D. Dihydrofolate reductase
Answer: B
Rationale: PKU results from deficiency of phenylalanine hydroxylase, causing
accumulation of phenylalanine and neurologic damage.
4. A patient with chronic alcoholism develops confusion, ophthalmoplegia, and
ataxia. Which brain structure is most affected?
A. Caudate nucleus
B. Mammillary bodies
C. Substantia nigra
D. Red nucleus
Answer: B
Rationale: Wernicke encephalopathy affects mammillary bodies due to thiamine
deficiency.
5. A drug decreases preload and causes venodilation. Which class does it belong
to?
A. Beta blockers
B. ACE inhibitors
C. Nitrates
D. Calcium channel blockers
Answer: C
Rationale: Nitrates primarily cause venodilation, reducing preload and myocardial oxygen
demand.
6. A newborn presents with ambiguous genitalia and salt-wasting crisis. Which
enzyme is most likely deficient?
, A. 21-hydroxylase
B. 17α-hydroxylase
C. 11β-hydroxylase
D. Aromatase
Answer: A
Rationale: 21-hydroxylase deficiency causes congenital adrenal hyperplasia with salt
wasting and virilization.
7. Which virus is most likely to cause erythema infectiosum?
A. Parvovirus B19
B. HHV-6
C. Measles virus
D. Rubella virus
Answer: A
Rationale: Parvovirus B19 causes fifth disease characterized by slapped-cheek rash.
8. A patient has difficulty abducting the arm beyond 15 degrees. Which muscle is
most likely injured?
A. Supraspinatus
B. Deltoid
C. Infraspinatus
D. Teres major
Answer: B
Rationale: The deltoid muscle is responsible for arm abduction beyond the initial 15
degrees.
9. Which neurotransmitter is decreased in Parkinson disease?
A. GABA
B. Acetylcholine
C. Dopamine
D. Serotonin
