NBME CBSE (USMLE Step 1) Exam Comprehensive Test Bank
300 Questions with Answers and Rationales | 2026-2027
Edition
EXAM OVERVIEW
The Comprehensive Basic Science Examination (CBSE) is a standardized exam
offered by the National Board of Medical Examiners (NBME) that approximates
the content and format of the USMLE Step 1 exam . It assesses understanding and
application of basic science principles underlying health, disease, and therapy .
Exam Format:
• 190 scored items
• Content mirrors USMLE Step 1 specifications
• Results correlate well with Step 1 performance
• Score equivalents: e.g., 70% correct ≈ 200 Step 1 equivalent
,SECTION 1: GENERAL PRINCIPLES (12-16%)
Questions 1-50
Q1. A 45-year-old male with a 20-pack-year smoking history presents with a 3-
month history of progressive dyspnea and non-productive cough. Chest X-ray
shows bilateral interstitial infiltrates. A biopsy reveals intra-alveolar
accumulation of large, foamy macrophages with periodic acid-Schiff (PAS)-
positive granules. Which of the following is the most likely diagnosis?
A) Desquamative interstitial pneumonia (DIP)
B) Usual interstitial pneumonia (UIP)
C) Cryptogenic organizing pneumonia (COP)
D) Respiratory bronchiolitis-interstitial lung disease (RB-ILD)
Answer: A) Desquamative interstitial pneumonia (DIP)
Rationale: DIP is characterized by the intra-alveolar accumulation of large, foamy
macrophages that are PAS-positive. It is most commonly associated with smoking,
and the "desquamation" refers to the intra-alveolar accumulation of
macrophages, not to actual desquamation of pneumocytes. UIP (B) is associated
with honeycombing and fibroblastic foci; COP (C) shows plugs of granulation
tissue within alveolar ducts; RB-ILD (D) is also smoking-associated but shows
pigmented macrophages in bronchioles rather than diffuse alveolar accumulation
.
Q2. A 30-year-old female presents with fatigue, malaise, and a malar rash.
Laboratory testing reveals ANA positivity and anti-dsDNA antibodies. Which of
the following mechanisms best explains the pathogenesis of this disease?
A) Type I hypersensitivity reaction
B) Type II hypersensitivity reaction
C) Type III hypersensitivity reaction
D) Type IV hypersensitivity reaction
Answer: C) Type III hypersensitivity reaction
,Rationale: Systemic lupus erythematosus (SLE) is characterized by type III
hypersensitivity reactions where immune complexes (antibody-antigen
complexes) deposit in tissues, leading to complement activation and
inflammation. The anti-dsDNA antibodies are classic for SLE and form immune
complexes that deposit in the kidney, skin, and other organs. Type I (A) is
allergic/anaphylactic; Type II (B) is antibody-mediated cytotoxicity; Type IV (D) is
delayed-type hypersensitivity (T-cell mediated) .
Q3. A 65-year-old male with a history of hypertension presents with chest pain.
ECG shows ST-segment elevation in leads II, III, and aVF. Which coronary artery
is most likely occluded?
A) Left anterior descending artery
B) Right coronary artery
C) Left circumflex artery
D) Left main coronary artery
Answer: B) Right coronary artery
Rationale: ST-elevation in leads II, III, and aVF indicates an inferior wall
myocardial infarction, which is most commonly caused by occlusion of the right
coronary artery (RCA). The LAD (A) supplies the anterior wall (leads V1-V4); the
LCx (C) supplies the lateral wall (leads I, aVL, V5-V6); the left main (D) supplies the
entire left ventricular distribution and usually causes extensive infarction with
cardiogenic shock .
Q4. A 22-year-old male presents with a painless, firm, rubbery lymph node in
the neck. Biopsy shows Reed-Sternberg cells. Which of the following is the most
likely diagnosis?
A) Non-Hodgkin lymphoma
B) Hodgkin lymphoma
C) Burkitt lymphoma
D) Follicular lymphoma
Answer: B) Hodgkin lymphoma
, Rationale: Reed-Sternberg cells (large, binucleated or multinucleated cells with
prominent nucleoli) are pathognomonic for Hodgkin lymphoma. They are typically
CD15+ and CD30+. Non-Hodgkin lymphoma (A) does not show Reed-Sternberg
cells; Burkitt lymphoma (C) shows "starry sky" appearance; Follicular lymphoma
(D) shows follicular pattern with Bcl-2 positivity .
Q5. A 55-year-old female with a history of smoking presents with a 2-month
history of hoarseness. Laryngoscopy reveals a lesion on the vocal cord. Biopsy
shows invasive squamous cell carcinoma. Which of the following is the most
important risk factor?
A) Alcohol consumption
B) Tobacco smoking
C) HPV infection
D) GERD
Answer: B) Tobacco smoking
Rationale: Tobacco smoking is the most important risk factor for squamous cell
carcinoma of the larynx. The combination of tobacco and alcohol (A) has a
synergistic effect, but smoking alone is the dominant risk factor. HPV (C) is
associated with oropharyngeal cancer, not laryngeal cancer; GERD (D) is a risk
factor but less significant .
Q6. A 60-year-old male with chronic obstructive pulmonary disease (COPD)
presents with worsening dyspnea and cough. Chest X-ray shows hyperinflation
and a bulla. Which of the following pathological findings is most characteristic
of emphysema?
A) Goblet cell hyperplasia
B) Mucous gland hypertrophy
C) Alveolar wall destruction
D) Bronchiolar fibrosis
Answer: C) Alveolar wall destruction
300 Questions with Answers and Rationales | 2026-2027
Edition
EXAM OVERVIEW
The Comprehensive Basic Science Examination (CBSE) is a standardized exam
offered by the National Board of Medical Examiners (NBME) that approximates
the content and format of the USMLE Step 1 exam . It assesses understanding and
application of basic science principles underlying health, disease, and therapy .
Exam Format:
• 190 scored items
• Content mirrors USMLE Step 1 specifications
• Results correlate well with Step 1 performance
• Score equivalents: e.g., 70% correct ≈ 200 Step 1 equivalent
,SECTION 1: GENERAL PRINCIPLES (12-16%)
Questions 1-50
Q1. A 45-year-old male with a 20-pack-year smoking history presents with a 3-
month history of progressive dyspnea and non-productive cough. Chest X-ray
shows bilateral interstitial infiltrates. A biopsy reveals intra-alveolar
accumulation of large, foamy macrophages with periodic acid-Schiff (PAS)-
positive granules. Which of the following is the most likely diagnosis?
A) Desquamative interstitial pneumonia (DIP)
B) Usual interstitial pneumonia (UIP)
C) Cryptogenic organizing pneumonia (COP)
D) Respiratory bronchiolitis-interstitial lung disease (RB-ILD)
Answer: A) Desquamative interstitial pneumonia (DIP)
Rationale: DIP is characterized by the intra-alveolar accumulation of large, foamy
macrophages that are PAS-positive. It is most commonly associated with smoking,
and the "desquamation" refers to the intra-alveolar accumulation of
macrophages, not to actual desquamation of pneumocytes. UIP (B) is associated
with honeycombing and fibroblastic foci; COP (C) shows plugs of granulation
tissue within alveolar ducts; RB-ILD (D) is also smoking-associated but shows
pigmented macrophages in bronchioles rather than diffuse alveolar accumulation
.
Q2. A 30-year-old female presents with fatigue, malaise, and a malar rash.
Laboratory testing reveals ANA positivity and anti-dsDNA antibodies. Which of
the following mechanisms best explains the pathogenesis of this disease?
A) Type I hypersensitivity reaction
B) Type II hypersensitivity reaction
C) Type III hypersensitivity reaction
D) Type IV hypersensitivity reaction
Answer: C) Type III hypersensitivity reaction
,Rationale: Systemic lupus erythematosus (SLE) is characterized by type III
hypersensitivity reactions where immune complexes (antibody-antigen
complexes) deposit in tissues, leading to complement activation and
inflammation. The anti-dsDNA antibodies are classic for SLE and form immune
complexes that deposit in the kidney, skin, and other organs. Type I (A) is
allergic/anaphylactic; Type II (B) is antibody-mediated cytotoxicity; Type IV (D) is
delayed-type hypersensitivity (T-cell mediated) .
Q3. A 65-year-old male with a history of hypertension presents with chest pain.
ECG shows ST-segment elevation in leads II, III, and aVF. Which coronary artery
is most likely occluded?
A) Left anterior descending artery
B) Right coronary artery
C) Left circumflex artery
D) Left main coronary artery
Answer: B) Right coronary artery
Rationale: ST-elevation in leads II, III, and aVF indicates an inferior wall
myocardial infarction, which is most commonly caused by occlusion of the right
coronary artery (RCA). The LAD (A) supplies the anterior wall (leads V1-V4); the
LCx (C) supplies the lateral wall (leads I, aVL, V5-V6); the left main (D) supplies the
entire left ventricular distribution and usually causes extensive infarction with
cardiogenic shock .
Q4. A 22-year-old male presents with a painless, firm, rubbery lymph node in
the neck. Biopsy shows Reed-Sternberg cells. Which of the following is the most
likely diagnosis?
A) Non-Hodgkin lymphoma
B) Hodgkin lymphoma
C) Burkitt lymphoma
D) Follicular lymphoma
Answer: B) Hodgkin lymphoma
, Rationale: Reed-Sternberg cells (large, binucleated or multinucleated cells with
prominent nucleoli) are pathognomonic for Hodgkin lymphoma. They are typically
CD15+ and CD30+. Non-Hodgkin lymphoma (A) does not show Reed-Sternberg
cells; Burkitt lymphoma (C) shows "starry sky" appearance; Follicular lymphoma
(D) shows follicular pattern with Bcl-2 positivity .
Q5. A 55-year-old female with a history of smoking presents with a 2-month
history of hoarseness. Laryngoscopy reveals a lesion on the vocal cord. Biopsy
shows invasive squamous cell carcinoma. Which of the following is the most
important risk factor?
A) Alcohol consumption
B) Tobacco smoking
C) HPV infection
D) GERD
Answer: B) Tobacco smoking
Rationale: Tobacco smoking is the most important risk factor for squamous cell
carcinoma of the larynx. The combination of tobacco and alcohol (A) has a
synergistic effect, but smoking alone is the dominant risk factor. HPV (C) is
associated with oropharyngeal cancer, not laryngeal cancer; GERD (D) is a risk
factor but less significant .
Q6. A 60-year-old male with chronic obstructive pulmonary disease (COPD)
presents with worsening dyspnea and cough. Chest X-ray shows hyperinflation
and a bulla. Which of the following pathological findings is most characteristic
of emphysema?
A) Goblet cell hyperplasia
B) Mucous gland hypertrophy
C) Alveolar wall destruction
D) Bronchiolar fibrosis
Answer: C) Alveolar wall destruction
