USMLE Step 1 practice Exam With
Actual 120 Questions & Verified
Answers,Plus Rationales/Expert
Verified For Guaranteed Pass Graded
A+/ 2025/2026 /Latest Update/Instant
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1. A 26-year-old male presents with muscle cramps and vague paresthesias. His serum
potassium is 2.9 mEq/L (normal 3.5–5.0). Which shift best explains hypokalemia from
β2-adrenergic agonist use?
A. Increased renal excretion of K+
B. Increased cellular uptake of K+ via Na+/K+ ATPase stimulation
C. Increased release of K+ from cells
D. Decreased renal reabsorption of K+ in the distal tubule
E. Increased gastrointestinal loss of K+
Answer: B. Increased cellular uptake of K+ via Na+/K+ ATPase stimulation
Rationale: β2-agonists (eg, albuterol) stimulate Na+/K+ ATPase activity in skeletal
muscle, driving K+ into cells and lowering serum potassium.
2. A 3-year-old child has recurrent otitis media. The organism is a gram-negative
coccobacillus that produces a polysaccharide capsule and requires factors X and V to
grow. Which organism is most likely?
A. Streptococcus pneumoniae
B. Haemophilus influenzae type b
C. Moraxella catarrhalis
D. Staphylococcus aureus
E. Neisseria meningitidis
Answer: B. Haemophilus influenzae type b
Rationale: H. influenzae (encapsulated strains like type b) are gram-negative
coccobacilli; some strains require both hemin (X) and NAD (V) for growth.
3. Which enzyme deficiency causes accumulation of branched-chain amino acids and
leads to maple syrup urine disease?
, A. Phenylalanine hydroxylase
B. Branched-chain α-ketoacid dehydrogenase
C. Homogentisate oxidase
D. Medium-chain acyl-CoA dehydrogenase
E. Ornithine transcarbamylase
Answer: B. Branched-chain α-ketoacid dehydrogenase
Rationale: Deficiency in branched-chain α-ketoacid dehydrogenase (BCKD) impairs
catabolism of leucine, isoleucine, and valine causing maple syrup urine disease.
4. A patient with atrial fibrillation is started on warfarin. Which lab test best monitors
warfarin effect?
A. aPTT
B. INR (prothrombin time standardized)
C. Platelet count
D. Bleeding time
E. Fibrinogen level
Answer: B. INR (prothrombin time standardized)
Rationale: Warfarin inhibits vitamin K-dependent clotting factors (II, VII, IX, X);
PT/INR reflects extrinsic pathway activity and is used to monitor warfarin.
5. A drug that slows AV node conduction and prolongs PR interval most likely acts by
which mechanism?
A. Blocking L-type Ca2+ channels
B. Blocking Na+ channels in phase 0 of ventricular myocyte
C. Increasing funny current (If) in SA node
D. Activating β-adrenergic receptors
E. Inhibiting acetylcholinesterase
Answer: A. Blocking L-type Ca2+ channels
Rationale: AV node conduction depends on Ca2+ currents; Ca2+ channel blockers
(eg, verapamil, diltiazem) slow AV conduction and prolong PR interval.
6. A newborn has a holosystolic murmur, failure to thrive, and frequent pulmonary
infections. Which congenital heart defect is most likely?
A. Ventricular septal defect (VSD)
B. Atrial septal defect (ASD)
C. Patent ductus arteriosus (PDA)
D. Tetralogy of Fallot
E. Transposition of the great arteries
Answer: A. Ventricular septal defect (VSD)
Rationale: VSD causes left-to-right shunt producing holosystolic murmur, increased
pulmonary blood flow, and symptoms like failure to thrive and recurrent
respiratory infections.
,7. Which metabolic pathway occurs in the mitochondrial matrix and yields NADH and
FADH2 for oxidative phosphorylation?
A. Glycolysis
B. Fatty acid β-oxidation
C. Pentose phosphate pathway
D. HMP shunt
E. Cytosolic pyruvate carboxylation
Answer: B. Fatty acid β-oxidation
Rationale: β-oxidation of fatty acids occurs in mitochondria producing acetyl-CoA,
NADH, and FADH2 for the electron transport chain.
8. A 45-year-old male with hyperlipidemia is given a statin. Statins reduce LDL primarily
by:
A. Inhibiting intestinal cholesterol absorption
B. Upregulating hepatic LDL receptors via HMG-CoA reductase inhibition
C. Activating LCAT in plasma
D. Inhibiting CETP activity
E. Increasing VLDL secretion from liver
Answer: B. Upregulating hepatic LDL receptors via HMG-CoA reductase inhibition
Rationale: Statins inhibit HMG-CoA reductase → decreased cholesterol synthesis →
upregulation of LDL receptors on hepatocytes → increased LDL clearance.
9. A patient has prolonged muscle contraction after administration of succinylcholine.
Which enzyme deficiency causes prolonged effect?
A. Acetylcholinesterase deficiency
B. Butyrylcholinesterase (pseudocholinesterase) deficiency
C. Monoamine oxidase deficiency
D. Choline acetyltransferase deficiency
E. Carnitine deficiency
Answer: B. Butyrylcholinesterase (pseudocholinesterase) deficiency
Rationale: Succinylcholine is metabolized by plasma butyrylcholinesterase;
deficiency leads to prolonged neuromuscular blockade.
10. A patient presents with nephrotic syndrome and effacement of podocyte foot
processes on electron microscopy. Which disease is most consistent?
A. Minimal change disease
B. Focal segmental glomerulosclerosis
C. Membranous nephropathy
D. IgA nephropathy
E. Post-streptococcal GN
Answer: A. Minimal change disease
, Rationale: Minimal change disease shows diffuse podocyte foot process effacement
on EM and presents with nephrotic syndrome, especially in children.
11. Which vitamin deficiency causes night blindness and xerosis of conjunctiva?
A. Vitamin C
B. Vitamin A
C. Vitamin D
D. Vitamin E
E. Vitamin K
Answer: B. Vitamin A
Rationale: Vitamin A is essential for formation of rhodopsin and maintenance of
epithelial tissues; deficiency causes night blindness and conjunctival xerosis.
12. A thymoma is associated with which autoimmune condition that presents with
muscle weakness improving after rest?
A. Myasthenia gravis
B. Lambert-Eaton myasthenic syndrome
C. Multiple sclerosis
D. Guillain-Barré syndrome
E. Amyotrophic lateral sclerosis
Answer: A. Myasthenia gravis
Rationale: Myasthenia gravis is often associated with thymoma and caused by
antibodies to the postsynaptic ACh receptor, producing fatigable weakness that
improves with rest.
13. A patient with chronic alcohol use has increased anion gap metabolic acidosis and
elevated β-hydroxybutyrate. Which ratio change explains this?
A. Increased NAD+/NADH ratio
B. Decreased NAD+/NADH ratio
C. Increased ATP/ADP ratio
D. Increased FAD/FADH2 ratio
E. Decreased AMP/ATP ratio
Answer: B. Decreased NAD+/NADH ratio
Rationale: Alcohol metabolism (ethanol → acetaldehyde → acetate) produces
NADH, lowering NAD+/NADH and favoring lactate and ketone formation leading to
lactic acidosis and ketosis.
14. Which bacterial structure is primarily responsible for antigenic variation that allows
immune evasion and recurrent infection in Neisseria gonorrhoeae?
A. Capsule polysaccharide
B. Lipooligosaccharide (LOS) core changes
C. Type IV pili (pilin) antigenic variation
D. Peptidoglycan cross-link alteration
Actual 120 Questions & Verified
Answers,Plus Rationales/Expert
Verified For Guaranteed Pass Graded
A+/ 2025/2026 /Latest Update/Instant
Download Pdf
1. A 26-year-old male presents with muscle cramps and vague paresthesias. His serum
potassium is 2.9 mEq/L (normal 3.5–5.0). Which shift best explains hypokalemia from
β2-adrenergic agonist use?
A. Increased renal excretion of K+
B. Increased cellular uptake of K+ via Na+/K+ ATPase stimulation
C. Increased release of K+ from cells
D. Decreased renal reabsorption of K+ in the distal tubule
E. Increased gastrointestinal loss of K+
Answer: B. Increased cellular uptake of K+ via Na+/K+ ATPase stimulation
Rationale: β2-agonists (eg, albuterol) stimulate Na+/K+ ATPase activity in skeletal
muscle, driving K+ into cells and lowering serum potassium.
2. A 3-year-old child has recurrent otitis media. The organism is a gram-negative
coccobacillus that produces a polysaccharide capsule and requires factors X and V to
grow. Which organism is most likely?
A. Streptococcus pneumoniae
B. Haemophilus influenzae type b
C. Moraxella catarrhalis
D. Staphylococcus aureus
E. Neisseria meningitidis
Answer: B. Haemophilus influenzae type b
Rationale: H. influenzae (encapsulated strains like type b) are gram-negative
coccobacilli; some strains require both hemin (X) and NAD (V) for growth.
3. Which enzyme deficiency causes accumulation of branched-chain amino acids and
leads to maple syrup urine disease?
, A. Phenylalanine hydroxylase
B. Branched-chain α-ketoacid dehydrogenase
C. Homogentisate oxidase
D. Medium-chain acyl-CoA dehydrogenase
E. Ornithine transcarbamylase
Answer: B. Branched-chain α-ketoacid dehydrogenase
Rationale: Deficiency in branched-chain α-ketoacid dehydrogenase (BCKD) impairs
catabolism of leucine, isoleucine, and valine causing maple syrup urine disease.
4. A patient with atrial fibrillation is started on warfarin. Which lab test best monitors
warfarin effect?
A. aPTT
B. INR (prothrombin time standardized)
C. Platelet count
D. Bleeding time
E. Fibrinogen level
Answer: B. INR (prothrombin time standardized)
Rationale: Warfarin inhibits vitamin K-dependent clotting factors (II, VII, IX, X);
PT/INR reflects extrinsic pathway activity and is used to monitor warfarin.
5. A drug that slows AV node conduction and prolongs PR interval most likely acts by
which mechanism?
A. Blocking L-type Ca2+ channels
B. Blocking Na+ channels in phase 0 of ventricular myocyte
C. Increasing funny current (If) in SA node
D. Activating β-adrenergic receptors
E. Inhibiting acetylcholinesterase
Answer: A. Blocking L-type Ca2+ channels
Rationale: AV node conduction depends on Ca2+ currents; Ca2+ channel blockers
(eg, verapamil, diltiazem) slow AV conduction and prolong PR interval.
6. A newborn has a holosystolic murmur, failure to thrive, and frequent pulmonary
infections. Which congenital heart defect is most likely?
A. Ventricular septal defect (VSD)
B. Atrial septal defect (ASD)
C. Patent ductus arteriosus (PDA)
D. Tetralogy of Fallot
E. Transposition of the great arteries
Answer: A. Ventricular septal defect (VSD)
Rationale: VSD causes left-to-right shunt producing holosystolic murmur, increased
pulmonary blood flow, and symptoms like failure to thrive and recurrent
respiratory infections.
,7. Which metabolic pathway occurs in the mitochondrial matrix and yields NADH and
FADH2 for oxidative phosphorylation?
A. Glycolysis
B. Fatty acid β-oxidation
C. Pentose phosphate pathway
D. HMP shunt
E. Cytosolic pyruvate carboxylation
Answer: B. Fatty acid β-oxidation
Rationale: β-oxidation of fatty acids occurs in mitochondria producing acetyl-CoA,
NADH, and FADH2 for the electron transport chain.
8. A 45-year-old male with hyperlipidemia is given a statin. Statins reduce LDL primarily
by:
A. Inhibiting intestinal cholesterol absorption
B. Upregulating hepatic LDL receptors via HMG-CoA reductase inhibition
C. Activating LCAT in plasma
D. Inhibiting CETP activity
E. Increasing VLDL secretion from liver
Answer: B. Upregulating hepatic LDL receptors via HMG-CoA reductase inhibition
Rationale: Statins inhibit HMG-CoA reductase → decreased cholesterol synthesis →
upregulation of LDL receptors on hepatocytes → increased LDL clearance.
9. A patient has prolonged muscle contraction after administration of succinylcholine.
Which enzyme deficiency causes prolonged effect?
A. Acetylcholinesterase deficiency
B. Butyrylcholinesterase (pseudocholinesterase) deficiency
C. Monoamine oxidase deficiency
D. Choline acetyltransferase deficiency
E. Carnitine deficiency
Answer: B. Butyrylcholinesterase (pseudocholinesterase) deficiency
Rationale: Succinylcholine is metabolized by plasma butyrylcholinesterase;
deficiency leads to prolonged neuromuscular blockade.
10. A patient presents with nephrotic syndrome and effacement of podocyte foot
processes on electron microscopy. Which disease is most consistent?
A. Minimal change disease
B. Focal segmental glomerulosclerosis
C. Membranous nephropathy
D. IgA nephropathy
E. Post-streptococcal GN
Answer: A. Minimal change disease
, Rationale: Minimal change disease shows diffuse podocyte foot process effacement
on EM and presents with nephrotic syndrome, especially in children.
11. Which vitamin deficiency causes night blindness and xerosis of conjunctiva?
A. Vitamin C
B. Vitamin A
C. Vitamin D
D. Vitamin E
E. Vitamin K
Answer: B. Vitamin A
Rationale: Vitamin A is essential for formation of rhodopsin and maintenance of
epithelial tissues; deficiency causes night blindness and conjunctival xerosis.
12. A thymoma is associated with which autoimmune condition that presents with
muscle weakness improving after rest?
A. Myasthenia gravis
B. Lambert-Eaton myasthenic syndrome
C. Multiple sclerosis
D. Guillain-Barré syndrome
E. Amyotrophic lateral sclerosis
Answer: A. Myasthenia gravis
Rationale: Myasthenia gravis is often associated with thymoma and caused by
antibodies to the postsynaptic ACh receptor, producing fatigable weakness that
improves with rest.
13. A patient with chronic alcohol use has increased anion gap metabolic acidosis and
elevated β-hydroxybutyrate. Which ratio change explains this?
A. Increased NAD+/NADH ratio
B. Decreased NAD+/NADH ratio
C. Increased ATP/ADP ratio
D. Increased FAD/FADH2 ratio
E. Decreased AMP/ATP ratio
Answer: B. Decreased NAD+/NADH ratio
Rationale: Alcohol metabolism (ethanol → acetaldehyde → acetate) produces
NADH, lowering NAD+/NADH and favoring lactate and ketone formation leading to
lactic acidosis and ketosis.
14. Which bacterial structure is primarily responsible for antigenic variation that allows
immune evasion and recurrent infection in Neisseria gonorrhoeae?
A. Capsule polysaccharide
B. Lipooligosaccharide (LOS) core changes
C. Type IV pili (pilin) antigenic variation
D. Peptidoglycan cross-link alteration
