First Aid USMLE Step 1 Questions and
Answers latest Updated Rated A+
Question 1
A 25-year-old man presents with fatigue, pallor, and occasional nosebleeds.
Laboratory studies show a hemoglobin of 8 g/dL, platelet count of 60,000/µL,
and a white blood cell count of 2,500/µL. A bone marrow biopsy shows
hypocellularity. Which of the following is the most likely diagnosis?
A. Acute myeloid leukemia
B. Aplastic anemia
C. Iron deficiency anemia
D. Myelodysplastic syndrome
E. Hemolytic anemia
Answer: B. Aplastic anemia
Explanation: Aplastic anemia presents with pancytopenia (low RBCs, WBCs,
and platelets) and a hypocellular bone marrow. Unlike leukemia or
myelodysplastic syndrome, there is no abnormal proliferation of blasts or
dysplasia. Iron deficiency anemia causes microcytic anemia without affecting
all cell lines. Hemolytic anemia typically shows elevated reticulocytes.
Question 2
A 60-year-old woman presents with sudden-onset severe chest pain radiating
to her back. She has a history of hypertension. On examination, her blood
pressure is 180/100 mmHg in the right arm and 150/90 mmHg in the left
arm. Which of the following is the most likely diagnosis?
A. Acute myocardial infarction
B. Pulmonary embolism
C. Aortic dissection
D. Pericarditis
E. Pneumothorax
Answer: C. Aortic dissection
Explanation: Aortic dissection classically presents with sudden, severe
chest or back pain, often with blood pressure discrepancies between arms.
Hypertension is a major risk factor. Myocardial infarction pain is usually
substernal and may radiate to the arm or jaw. Pulmonary embolism often
,causes dyspnea and pleuritic chest pain.
Question 3
A 35-year-old man presents with polyuria and polydipsia. Fasting plasma
glucose is 220 mg/dL. Hemoglobin A1c is 9%. Which of the following is the
most likely underlying pathophysiology?
A. Autoimmune destruction of pancreatic beta cells
B. Insulin resistance in peripheral tissues
C. Excess glucagon secretion
D. Deficiency of incretin hormones
E. Impaired renal glucose reabsorption
Answer: A. Autoimmune destruction of pancreatic beta cells
Explanation: This patient has classic features of type 1 diabetes mellitus,
which is caused by autoimmune destruction of pancreatic beta cells, leading
to insulin deficiency. Insulin resistance is the main issue in type 2 diabetes.
Excess glucagon or impaired renal glucose reabsorption does not account for
the elevated glucose and ketone-prone state.
Question 4
A 50-year-old man presents with jaundice, dark urine, and fatigue.
Laboratory tests show elevated AST > ALT, elevated bilirubin, and positive
hepatitis B surface antigen. Which of the following complications is he at
greatest risk for?
A. Acute pancreatitis
B. Hepatocellular carcinoma
C. Chronic kidney disease
D. Circulatory shock
E. Acute myocardial infarction
Answer: B. Hepatocellular carcinoma
Explanation: Chronic hepatitis B infection increases the risk of
hepatocellular carcinoma and cirrhosis. AST>ALT is often seen in alcoholic
liver disease, but in hepatitis, ALT can be higher initially. Acute pancreatitis
and myocardial infarction are unrelated. Kidney disease may occur with
hepatitis-related glomerulonephritis but is less common.
Question 5
A 28-year-old woman presents with a butterfly-shaped rash on her face that
worsens with sunlight exposure. She also reports joint pain and fatigue.
,Laboratory studies show positive ANA and anti-dsDNA antibodies. Which of
the following is the most likely diagnosis?
A. Rheumatoid arthritis
B. Systemic lupus erythematosus
C. Dermatomyositis
D. Psoriasis
E. Scleroderma
Answer: B. Systemic lupus erythematosus
Explanation: The malar rash, photosensitivity, joint pain, and positive ANA
and anti-dsDNA antibodies are classic for SLE. Rheumatoid arthritis primarily
affects joints without characteristic rash. Dermatomyositis presents with
heliotrope rash and proximal muscle weakness. Psoriasis has scaly plaques.
Scleroderma shows skin thickening and Raynaud’s phenomenon.
Question 6
A 65-year-old man presents with a tremor at rest, bradykinesia, and a
shuffling gait. MRI of the brain is unremarkable. Which of the following
neurotransmitter deficiencies is most likely involved?
A. Acetylcholine
B. Dopamine
C. GABA
D. Serotonin
E. Norepinephrine
Answer: B. Dopamine
Explanation: Parkinson disease is caused by degeneration of dopaminergic
neurons in the substantia nigra, leading to dopamine deficiency. Resting
tremor, bradykinesia, and rigidity are hallmark features. Acetylcholine is
relatively increased, contributing to tremor. GABA, serotonin, and
norepinephrine are not primary in Parkinson’s pathophysiology.
Question 7
A 22-year-old college student presents with fever, sore throat, and
generalized lymphadenopathy. Peripheral smear shows atypical
lymphocytes. Heterophile antibody test is positive. Which virus is most likely
responsible?
A. Cytomegalovirus
B. Epstein-Barr virus
C. Herpes simplex virus
, D. Human immunodeficiency virus
E. Influenza virus
Answer: B. Epstein-Barr virus
Explanation: Infectious mononucleosis caused by EBV presents with fever,
pharyngitis, lymphadenopathy, and atypical lymphocytes. Heterophile
antibody test (Monospot) is positive. CMV can cause a similar syndrome but
often heterophile negative. HSV causes mucocutaneous lesions. HIV causes
immunodeficiency over months to years.
Question 8
A 70-year-old man with chronic obstructive pulmonary disease presents with
progressive shortness of breath. Arterial blood gas shows PaO2 55 mmHg,
PaCO2 50 mmHg, pH 7.36. Which of the following is the acid-base disorder?
A. Respiratory acidosis, uncompensated
B. Respiratory acidosis, partially compensated
C. Metabolic acidosis, partially compensated
D. Metabolic alkalosis, fully compensated
E. Respiratory alkalosis, uncompensated
Answer: B. Respiratory acidosis, partially compensated
Explanation: Chronic COPD can cause CO2 retention, leading to respiratory
acidosis. The pH is near normal due to renal compensation (bicarbonate
retention), making it partially compensated. Metabolic acidosis or alkalosis
does not fit the lab values. Respiratory alkalosis would have low PaCO2.
Question 9
A 5-year-old boy presents with frequent bacterial infections and absence of
pus formation. Laboratory studies show normal T-cell counts but very low B-
cell counts. Which of the following is the most likely diagnosis?
A. Severe combined immunodeficiency
B. Bruton's agammaglobulinemia
C. DiGeorge syndrome
D. Hyper IgM syndrome
E. Common variable immunodeficiency
Answer: B. Bruton's agammaglobulinemia
Explanation: Bruton's X-linked agammaglobulinemia is characterized by
absence of mature B cells and very low immunoglobulins. T-cell function is
normal. Patients present with recurrent bacterial infections without pus
formation. SCID affects both T and B cells. DiGeorge involves T-cell
Answers latest Updated Rated A+
Question 1
A 25-year-old man presents with fatigue, pallor, and occasional nosebleeds.
Laboratory studies show a hemoglobin of 8 g/dL, platelet count of 60,000/µL,
and a white blood cell count of 2,500/µL. A bone marrow biopsy shows
hypocellularity. Which of the following is the most likely diagnosis?
A. Acute myeloid leukemia
B. Aplastic anemia
C. Iron deficiency anemia
D. Myelodysplastic syndrome
E. Hemolytic anemia
Answer: B. Aplastic anemia
Explanation: Aplastic anemia presents with pancytopenia (low RBCs, WBCs,
and platelets) and a hypocellular bone marrow. Unlike leukemia or
myelodysplastic syndrome, there is no abnormal proliferation of blasts or
dysplasia. Iron deficiency anemia causes microcytic anemia without affecting
all cell lines. Hemolytic anemia typically shows elevated reticulocytes.
Question 2
A 60-year-old woman presents with sudden-onset severe chest pain radiating
to her back. She has a history of hypertension. On examination, her blood
pressure is 180/100 mmHg in the right arm and 150/90 mmHg in the left
arm. Which of the following is the most likely diagnosis?
A. Acute myocardial infarction
B. Pulmonary embolism
C. Aortic dissection
D. Pericarditis
E. Pneumothorax
Answer: C. Aortic dissection
Explanation: Aortic dissection classically presents with sudden, severe
chest or back pain, often with blood pressure discrepancies between arms.
Hypertension is a major risk factor. Myocardial infarction pain is usually
substernal and may radiate to the arm or jaw. Pulmonary embolism often
,causes dyspnea and pleuritic chest pain.
Question 3
A 35-year-old man presents with polyuria and polydipsia. Fasting plasma
glucose is 220 mg/dL. Hemoglobin A1c is 9%. Which of the following is the
most likely underlying pathophysiology?
A. Autoimmune destruction of pancreatic beta cells
B. Insulin resistance in peripheral tissues
C. Excess glucagon secretion
D. Deficiency of incretin hormones
E. Impaired renal glucose reabsorption
Answer: A. Autoimmune destruction of pancreatic beta cells
Explanation: This patient has classic features of type 1 diabetes mellitus,
which is caused by autoimmune destruction of pancreatic beta cells, leading
to insulin deficiency. Insulin resistance is the main issue in type 2 diabetes.
Excess glucagon or impaired renal glucose reabsorption does not account for
the elevated glucose and ketone-prone state.
Question 4
A 50-year-old man presents with jaundice, dark urine, and fatigue.
Laboratory tests show elevated AST > ALT, elevated bilirubin, and positive
hepatitis B surface antigen. Which of the following complications is he at
greatest risk for?
A. Acute pancreatitis
B. Hepatocellular carcinoma
C. Chronic kidney disease
D. Circulatory shock
E. Acute myocardial infarction
Answer: B. Hepatocellular carcinoma
Explanation: Chronic hepatitis B infection increases the risk of
hepatocellular carcinoma and cirrhosis. AST>ALT is often seen in alcoholic
liver disease, but in hepatitis, ALT can be higher initially. Acute pancreatitis
and myocardial infarction are unrelated. Kidney disease may occur with
hepatitis-related glomerulonephritis but is less common.
Question 5
A 28-year-old woman presents with a butterfly-shaped rash on her face that
worsens with sunlight exposure. She also reports joint pain and fatigue.
,Laboratory studies show positive ANA and anti-dsDNA antibodies. Which of
the following is the most likely diagnosis?
A. Rheumatoid arthritis
B. Systemic lupus erythematosus
C. Dermatomyositis
D. Psoriasis
E. Scleroderma
Answer: B. Systemic lupus erythematosus
Explanation: The malar rash, photosensitivity, joint pain, and positive ANA
and anti-dsDNA antibodies are classic for SLE. Rheumatoid arthritis primarily
affects joints without characteristic rash. Dermatomyositis presents with
heliotrope rash and proximal muscle weakness. Psoriasis has scaly plaques.
Scleroderma shows skin thickening and Raynaud’s phenomenon.
Question 6
A 65-year-old man presents with a tremor at rest, bradykinesia, and a
shuffling gait. MRI of the brain is unremarkable. Which of the following
neurotransmitter deficiencies is most likely involved?
A. Acetylcholine
B. Dopamine
C. GABA
D. Serotonin
E. Norepinephrine
Answer: B. Dopamine
Explanation: Parkinson disease is caused by degeneration of dopaminergic
neurons in the substantia nigra, leading to dopamine deficiency. Resting
tremor, bradykinesia, and rigidity are hallmark features. Acetylcholine is
relatively increased, contributing to tremor. GABA, serotonin, and
norepinephrine are not primary in Parkinson’s pathophysiology.
Question 7
A 22-year-old college student presents with fever, sore throat, and
generalized lymphadenopathy. Peripheral smear shows atypical
lymphocytes. Heterophile antibody test is positive. Which virus is most likely
responsible?
A. Cytomegalovirus
B. Epstein-Barr virus
C. Herpes simplex virus
, D. Human immunodeficiency virus
E. Influenza virus
Answer: B. Epstein-Barr virus
Explanation: Infectious mononucleosis caused by EBV presents with fever,
pharyngitis, lymphadenopathy, and atypical lymphocytes. Heterophile
antibody test (Monospot) is positive. CMV can cause a similar syndrome but
often heterophile negative. HSV causes mucocutaneous lesions. HIV causes
immunodeficiency over months to years.
Question 8
A 70-year-old man with chronic obstructive pulmonary disease presents with
progressive shortness of breath. Arterial blood gas shows PaO2 55 mmHg,
PaCO2 50 mmHg, pH 7.36. Which of the following is the acid-base disorder?
A. Respiratory acidosis, uncompensated
B. Respiratory acidosis, partially compensated
C. Metabolic acidosis, partially compensated
D. Metabolic alkalosis, fully compensated
E. Respiratory alkalosis, uncompensated
Answer: B. Respiratory acidosis, partially compensated
Explanation: Chronic COPD can cause CO2 retention, leading to respiratory
acidosis. The pH is near normal due to renal compensation (bicarbonate
retention), making it partially compensated. Metabolic acidosis or alkalosis
does not fit the lab values. Respiratory alkalosis would have low PaCO2.
Question 9
A 5-year-old boy presents with frequent bacterial infections and absence of
pus formation. Laboratory studies show normal T-cell counts but very low B-
cell counts. Which of the following is the most likely diagnosis?
A. Severe combined immunodeficiency
B. Bruton's agammaglobulinemia
C. DiGeorge syndrome
D. Hyper IgM syndrome
E. Common variable immunodeficiency
Answer: B. Bruton's agammaglobulinemia
Explanation: Bruton's X-linked agammaglobulinemia is characterized by
absence of mature B cells and very low immunoglobulins. T-cell function is
normal. Patients present with recurrent bacterial infections without pus
formation. SCID affects both T and B cells. DiGeorge involves T-cell
