USMLE Step 1 Exam Practice Questions
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Rationale 2026 Q&A| Instant Download
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1. A 23-year-old medical student develops fatigue and pallor. Laboratory
studies show microcytic hypochromic anemia with decreased serum
ferritin. Which process is most directly impaired?
A. Hemoglobin synthesis due to decreased iron availability
B. DNA synthesis due to folate deficiency
C. Globin chain production due to ribosomal defect
D. Heme degradation due to bilirubin accumulation
A. Hemoglobin synthesis due to decreased iron availability
Iron deficiency reduces heme formation, leading to impaired
hemoglobin synthesis and resulting in microcytic hypochromic
anemia.
2. A patient has a mutation affecting mitochondrial ATP synthase. Which
process is most directly affected?
A. Glycolysis
B. Oxidative phosphorylation
C. Krebs cycle substrate-level phosphorylation
D. Fatty acid beta-oxidation initiation
B. Oxidative phosphorylation
, ATP synthase is essential for ATP production during oxidative
phosphorylation in mitochondria.
3. A newborn presents with prolonged jaundice and increased
unconjugated bilirubin. Which enzyme is most likely deficient?
A. UDP-glucuronyl transferase
B. Alkaline phosphatase
C. Lipoprotein lipase
D. Glucose-6-phosphatase
A. UDP-glucuronyl transferase
This enzyme conjugates bilirubin in the liver, and its deficiency causes
unconjugated hyperbilirubinemia.
4. A patient has ptosis, diplopia, and muscle weakness that worsens with
activity. Which antibody is most likely present?
A. Anti-acetylcholine receptor antibody
B. Anti-dsDNA antibody
C. Anti-centromere antibody
D. Anti-mitochondrial antibody
A. Anti-acetylcholine receptor antibody
Myasthenia gravis is caused by antibodies against acetylcholine
receptors at the neuromuscular junction.
5. Which hormone increases sodium reabsorption in the distal nephron
by increasing epithelial sodium channels?
A. Aldosterone
B. ADH
C. Atrial natriuretic peptide
D. Parathyroid hormone
A. Aldosterone
Aldosterone increases ENaC expression, enhancing sodium
reabsorption in the collecting duct.
, 6. A patient with COPD has chronic CO2 retention. What is the primary
compensatory change in the kidneys?
A. Increased bicarbonate excretion
B. Increased hydrogen ion secretion and bicarbonate reabsorption
C. Decreased ammoniagenesis
D. Decreased phosphate buffering
B. Increased hydrogen ion secretion and bicarbonate reabsorption
The kidneys compensate for respiratory acidosis by retaining
bicarbonate and secreting hydrogen ions.
7. A mutation affects collagen synthesis causing defective hydroxylation
of proline residues. Which vitamin deficiency is responsible?
A. Vitamin C
B. Vitamin D
C. Vitamin K
D. Vitamin B12
A. Vitamin C
Vitamin C is required for proline and lysine hydroxylation in collagen
synthesis.
8. A child presents with cystic fibrosis. Which transporter is defective?
A. Sodium-potassium ATPase
B. CFTR chloride channel
C. GLUT4 transporter
D. Aquaporin-2 channel
B. CFTR chloride channel
CFTR dysfunction leads to thick secretions due to impaired chloride
transport.
9. A patient has hypercalcemia and low phosphate. Which hormone is
most likely elevated?
A. Calcitonin
And Correct
Answers (Verified Answers) Plus
Rationale 2026 Q&A| Instant Download
1. A 23-year-old medical student develops fatigue and pallor. Laboratory
studies show microcytic hypochromic anemia with decreased serum
ferritin. Which process is most directly impaired?
A. Hemoglobin synthesis due to decreased iron availability
B. DNA synthesis due to folate deficiency
C. Globin chain production due to ribosomal defect
D. Heme degradation due to bilirubin accumulation
A. Hemoglobin synthesis due to decreased iron availability
Iron deficiency reduces heme formation, leading to impaired
hemoglobin synthesis and resulting in microcytic hypochromic
anemia.
2. A patient has a mutation affecting mitochondrial ATP synthase. Which
process is most directly affected?
A. Glycolysis
B. Oxidative phosphorylation
C. Krebs cycle substrate-level phosphorylation
D. Fatty acid beta-oxidation initiation
B. Oxidative phosphorylation
, ATP synthase is essential for ATP production during oxidative
phosphorylation in mitochondria.
3. A newborn presents with prolonged jaundice and increased
unconjugated bilirubin. Which enzyme is most likely deficient?
A. UDP-glucuronyl transferase
B. Alkaline phosphatase
C. Lipoprotein lipase
D. Glucose-6-phosphatase
A. UDP-glucuronyl transferase
This enzyme conjugates bilirubin in the liver, and its deficiency causes
unconjugated hyperbilirubinemia.
4. A patient has ptosis, diplopia, and muscle weakness that worsens with
activity. Which antibody is most likely present?
A. Anti-acetylcholine receptor antibody
B. Anti-dsDNA antibody
C. Anti-centromere antibody
D. Anti-mitochondrial antibody
A. Anti-acetylcholine receptor antibody
Myasthenia gravis is caused by antibodies against acetylcholine
receptors at the neuromuscular junction.
5. Which hormone increases sodium reabsorption in the distal nephron
by increasing epithelial sodium channels?
A. Aldosterone
B. ADH
C. Atrial natriuretic peptide
D. Parathyroid hormone
A. Aldosterone
Aldosterone increases ENaC expression, enhancing sodium
reabsorption in the collecting duct.
, 6. A patient with COPD has chronic CO2 retention. What is the primary
compensatory change in the kidneys?
A. Increased bicarbonate excretion
B. Increased hydrogen ion secretion and bicarbonate reabsorption
C. Decreased ammoniagenesis
D. Decreased phosphate buffering
B. Increased hydrogen ion secretion and bicarbonate reabsorption
The kidneys compensate for respiratory acidosis by retaining
bicarbonate and secreting hydrogen ions.
7. A mutation affects collagen synthesis causing defective hydroxylation
of proline residues. Which vitamin deficiency is responsible?
A. Vitamin C
B. Vitamin D
C. Vitamin K
D. Vitamin B12
A. Vitamin C
Vitamin C is required for proline and lysine hydroxylation in collagen
synthesis.
8. A child presents with cystic fibrosis. Which transporter is defective?
A. Sodium-potassium ATPase
B. CFTR chloride channel
C. GLUT4 transporter
D. Aquaporin-2 channel
B. CFTR chloride channel
CFTR dysfunction leads to thick secretions due to impaired chloride
transport.
9. A patient has hypercalcemia and low phosphate. Which hormone is
most likely elevated?
A. Calcitonin
