NBME CBSE USMLE Step 1 Exam 1
QUESTIONS AND CORRECT ANSWERS
(VERIFIED ANSWERS) PLUS RATIONALES
2025
1. A 24-year-old man has difficulty relaxing his muscles after contraction. He
has a family history of similar problems. The defect most likely involves
which of the following proteins?
A. Tropomyosin
B. Myotonin-protein kinase
C. Dystrophin
D. Calmodulin
E. Myosin light-chain kinase
Myotonic dystrophy is caused by a CTG trinucleotide repeat expansion in the
DMPK gene, affecting myotonin-protein kinase, leading to delayed muscle
relaxation.
, 2. A 67-year-old man with chronic cough is found to have squamous cell
carcinoma of the lung. The tumor produces a substance causing
hypercalcemia. Which substance is most likely responsible?
A. 1,25-dihydroxyvitamin D
B. Parathyroid hormone-related peptide (PTHrP)
C. Parathyroid hormone
D. Calcitonin
E. Interleukin-6
PTHrP is secreted by squamous cell carcinoma of the lung, leading to humoral
hypercalcemia of malignancy.
3. A 2-year-old girl presents with recurrent bacterial infections. Laboratory
studies reveal low levels of all immunoglobulins. Which of the following
processes is most likely defective?
A. Antigen processing by MHC I
B. B cell maturation
C. T cell receptor rearrangement
D. Cytokine production by macrophages
E. Class switching in plasma cells
A defect in B cell maturation, as seen in Bruton's agammaglobulinemia, results
in low levels of all immunoglobulin isotypes.
, 4. A 58-year-old woman with long-standing hypertension is prescribed a
thiazide diuretic. Which of the following is a common metabolic side effect?
A. Hyperkalemia
B. Metabolic acidosis
C. Hyperuricemia
D. Hypocalcemia
E. Hyponatremia
Thiazide diuretics decrease uric acid excretion, predisposing patients to
hyperuricemia and gout.
5. A 20-year-old college student is hospitalized with Neisseria meningitidis.
Which of the following bacterial components is most responsible for the
pathogenesis?
A. Pili
B. Lipooligosaccharide (LOS)
C. Porin proteins
D. IgA protease
E. Capsule
LOS acts as an endotoxin in Neisseria meningitidis, leading to sepsis and
inflammation.
, 6. A newborn has a heart murmur. Echocardiography reveals a ventricular
septal defect. Which of the following embryologic structures failed to
develop properly?
A. Truncus arteriosus
B. Membranous interventricular septum
C. Muscular interventricular septum
D. Endocardial cushions
E. Bulbus cordis
A defect in the membranous interventricular septum is the most common cause
of VSD.
7. A patient with cystic fibrosis presents with malabsorption. Which vitamin
deficiency is most likely?
A. Vitamin B1
B. Vitamin B6
C. Vitamin E
D. Vitamin C
E. Vitamin B12
CF leads to fat malabsorption, and vitamin E is a fat-soluble vitamin whose
deficiency can cause neurologic symptoms.
8. A researcher is studying muscle contraction. Which of the following
changes occurs when skeletal muscle contracts?
QUESTIONS AND CORRECT ANSWERS
(VERIFIED ANSWERS) PLUS RATIONALES
2025
1. A 24-year-old man has difficulty relaxing his muscles after contraction. He
has a family history of similar problems. The defect most likely involves
which of the following proteins?
A. Tropomyosin
B. Myotonin-protein kinase
C. Dystrophin
D. Calmodulin
E. Myosin light-chain kinase
Myotonic dystrophy is caused by a CTG trinucleotide repeat expansion in the
DMPK gene, affecting myotonin-protein kinase, leading to delayed muscle
relaxation.
, 2. A 67-year-old man with chronic cough is found to have squamous cell
carcinoma of the lung. The tumor produces a substance causing
hypercalcemia. Which substance is most likely responsible?
A. 1,25-dihydroxyvitamin D
B. Parathyroid hormone-related peptide (PTHrP)
C. Parathyroid hormone
D. Calcitonin
E. Interleukin-6
PTHrP is secreted by squamous cell carcinoma of the lung, leading to humoral
hypercalcemia of malignancy.
3. A 2-year-old girl presents with recurrent bacterial infections. Laboratory
studies reveal low levels of all immunoglobulins. Which of the following
processes is most likely defective?
A. Antigen processing by MHC I
B. B cell maturation
C. T cell receptor rearrangement
D. Cytokine production by macrophages
E. Class switching in plasma cells
A defect in B cell maturation, as seen in Bruton's agammaglobulinemia, results
in low levels of all immunoglobulin isotypes.
, 4. A 58-year-old woman with long-standing hypertension is prescribed a
thiazide diuretic. Which of the following is a common metabolic side effect?
A. Hyperkalemia
B. Metabolic acidosis
C. Hyperuricemia
D. Hypocalcemia
E. Hyponatremia
Thiazide diuretics decrease uric acid excretion, predisposing patients to
hyperuricemia and gout.
5. A 20-year-old college student is hospitalized with Neisseria meningitidis.
Which of the following bacterial components is most responsible for the
pathogenesis?
A. Pili
B. Lipooligosaccharide (LOS)
C. Porin proteins
D. IgA protease
E. Capsule
LOS acts as an endotoxin in Neisseria meningitidis, leading to sepsis and
inflammation.
, 6. A newborn has a heart murmur. Echocardiography reveals a ventricular
septal defect. Which of the following embryologic structures failed to
develop properly?
A. Truncus arteriosus
B. Membranous interventricular septum
C. Muscular interventricular septum
D. Endocardial cushions
E. Bulbus cordis
A defect in the membranous interventricular septum is the most common cause
of VSD.
7. A patient with cystic fibrosis presents with malabsorption. Which vitamin
deficiency is most likely?
A. Vitamin B1
B. Vitamin B6
C. Vitamin E
D. Vitamin C
E. Vitamin B12
CF leads to fat malabsorption, and vitamin E is a fat-soluble vitamin whose
deficiency can cause neurologic symptoms.
8. A researcher is studying muscle contraction. Which of the following
changes occurs when skeletal muscle contracts?
