NBME CBSE Exam Test Bank| Complete 200 Questions,
Correct Answers & Detailed Answers (USMLE Step 1)
Medical Examination 2026 Latest Update
EXAM OVERVIEW & KEY INFORMATION
The NBME Comprehensive Basic Science Examination (CBSE) is a standardized
assessment administered by the National Board of Medical Examiners (NBME) that
evaluates medical students' understanding of the basic sciences covered during the first
two years of medical school . The CBSE is used by medical schools as a placement
examination into clinical years and as a readiness assessment for USMLE Step 1 .
KEY EXAM INFORMATION:
• Number of Scored Items: 200 questions
• Content Coverage: Anatomy, physiology, pathology, pharmacology,
microbiology, biochemistry, immunology, genetics, behavioral sciences, and
biostatistics
• Score Reporting: Total Equated Percent Correct (EPC) score, representing the
percentage of content mastered
• Standard Error of Estimate (SEE): 4 points
• Exam Purpose: Assesses readiness for USMLE Step 1 and advancement to
clinical training
Question 1
A 40-year-old woman at 5 months' gestation comes to the physician for amniocentesis.
Results show a normal 46,XY karyotype of the fetus. Four months later, the newborn is
delivered. The infant is noted to have ambiguous genitalia. Which of the following is the
most likely cause?
A) Androgen insensitivity syndrome
B) 5-alpha-reductase deficiency
C) Congenital adrenal hyperplasia
D) Klinefelter syndrome
E) Turner syndrome
Correct Answer: C
Explanation: A 46,XY karyotype with ambiguous genitalia at birth suggests impaired
pg. 1
,androgen production or action. Congenital adrenal hyperplasia due to 21-hydroxylase
deficiency can cause virilization in females, but in a 46,XY male, impaired testosterone
synthesis can lead to undervirilization. The normal karyotype with female external
genitalia in a 46,XY individual suggests complete androgen insensitivity syndrome, but
ambiguous genitalia points to partial androgen insensitivity or 5-alpha-reductase
deficiency.
Question 2
Free purine and pyrimidine bases are reutilized in normal metabolism. In children with
Lesch-Nyhan syndrome who have intellectual disability, poor muscle coordination, and
self-mutilation tendencies, there is a defect in the salvage of which of the following pairs
of bases?
A) Adenine and thymine
B) Guanine and hypoxanthine
C) Guanine and uric acid
D) Uracil and cytosine
E) Xanthine and hypoxanthine
Correct Answer: B
Explanation: Lesch-Nyhan syndrome presents with intellectual disability, aggressive
behavior, self-mutilation, gout, and dystonia. The disorder is due to inactivating
mutations of hypoxanthine-guanine phosphoribosyltransferase (HGPRT), a key enzyme
in the purine salvage pathway, inherited in an X-linked recessive fashion. HGPRT
catalyzes the conversion of guanine to guanosine monophosphate and hypoxanthine to
inosine monophosphate. Patients with deficient HGPRT activity cannot salvage guanine
and hypoxanthine, developing increased levels of xanthine and uric acid. Hyperuricemia
in Lesch-Nyhan syndrome is treated with xanthine oxidase inhibitors such as
allopurinol .
Incorrect Answer Rationales:
• Adenine and thymine (Choice A) are purine and pyrimidine bases, respectively,
handled through distinct salvage pathways not commonly involved in a single
disease.
• Guanine and uric acid (Choice C) accumulation may occur, but uric acid
accumulation is secondary to hypoxanthine accumulation.
• Uracil and cytosine (Choice D) are pyrimidine nucleotides; pyrimidine salvage is
not affected by HGPRT mutations.
pg. 2
, • Defects of xanthine and hypoxanthine (Choice E) may result from HGPRT
defects; however, HGPRT dysfunction results in impaired hypoxanthine salvage
with excessive xanthine production, rather than impaired xanthine salvage .
Question 3
A 42-year-old man is struck by a motor vehicle. His only injury is a closed fracture of the
proximal tibia. Initial neurovascular examination shows no deficits. Twenty-four hours
later, he has increased leg pain and paresthesias in the dorsal space between the first
and second toes. Which of the following is the most likely diagnosis?
A) Compartment syndrome
B) Deep vein thrombosis
C) Fat embolism syndrome
D) Peroneal nerve palsy
E) Tibial nerve palsy
Correct Answer: A
Explanation: Compartment syndrome is a limb-threatening condition characterized
by increased pressure within a closed fascial compartment, compromising tissue
perfusion. Risk factors include fractures (especially tibial), crush injuries, and
reperfusion injury. Symptoms include pain out of proportion to injury, paresthesia,
pallor, pulselessness, and paralysis. Paresthesias in the first dorsal web space suggest
involvement of the deep peroneal nerve, which runs through the anterior compartment.
The diagnosis is confirmed by intracompartmental pressure monitoring.
Question 4
A 65-year-old woman has ascites. Which of the following additional findings indicates a
diagnosis of constrictive pericarditis rather than cirrhosis?
A) Edema of the lower extremities
B) Esophageal varices
C) Hypoalbuminemia
D) Hyponatremia
E) Increased jugular venous pressure
F) Splenomegaly
Correct Answer: E
Explanation: Increased jugular venous pressure (JVP) is an expected finding in
constrictive pericarditis (CP) and would not be found in patients with volume overload
secondary to cirrhosis. CP describes a pathologic state where the pericardium loses its
elasticity, impairing diastolic filling of the heart. During the normal cardiac cycle,
increased venous return to the right atrium and ventricle during inspiration leads to
pg. 3
, transient RV expansion. In CP, the rigid pericardium prevents this expansion, leading to
elevated JVP. Cirrhosis causes ascites through portal hypertension and
hypoalbuminemia, but JVP is typically normal. The differentiation between CP and
cirrhosis is critical because CP is potentially reversible with pericardiectomy .
Question 5
A 62-year-old man with a 40-pack-year smoking history presents with progressive
shortness of breath and a chronic cough. Physical examination reveals jugular venous
distention, peripheral edema, and a loud P2. Echocardiography shows right ventricular
hypertrophy. Which of the following is the most likely underlying cause?
A) Left-sided heart failure
B) Pulmonary arterial hypertension
C) Aortic stenosis
D) Mitral regurgitation
E) Pericardial constriction
Correct Answer: B
Explanation: Pulmonary arterial hypertension from chronic hypoxemia secondary to
COPD leads to cor pulmonale with right ventricular hypertrophy, jugular venous
distention, and a loud P2 .
Question 6
A 55-year-old woman with hypertension and type 2 diabetes mellitus is started on a new
medication. She returns 2 weeks later with a persistent dry cough. Her blood pressure is
well controlled. Which medication is most likely responsible?
A) Hydrochlorothiazide
B) Amlodipine
C) Lisinopril
D) Metoprolol
E) Losartan
Correct Answer: C
Explanation: ACE inhibitors (lisinopril) commonly cause a persistent dry cough due to
accumulation of bradykinin. Angiotensin receptor blockers (losartan) do not cause this
side effect .
Question 7
A 48-year-old man presents with sudden onset of severe "tearing" chest pain radiating
to his back. His blood pressure is 180/100 mmHg in the right arm and 140/80 mmHg in
pg. 4
Correct Answers & Detailed Answers (USMLE Step 1)
Medical Examination 2026 Latest Update
EXAM OVERVIEW & KEY INFORMATION
The NBME Comprehensive Basic Science Examination (CBSE) is a standardized
assessment administered by the National Board of Medical Examiners (NBME) that
evaluates medical students' understanding of the basic sciences covered during the first
two years of medical school . The CBSE is used by medical schools as a placement
examination into clinical years and as a readiness assessment for USMLE Step 1 .
KEY EXAM INFORMATION:
• Number of Scored Items: 200 questions
• Content Coverage: Anatomy, physiology, pathology, pharmacology,
microbiology, biochemistry, immunology, genetics, behavioral sciences, and
biostatistics
• Score Reporting: Total Equated Percent Correct (EPC) score, representing the
percentage of content mastered
• Standard Error of Estimate (SEE): 4 points
• Exam Purpose: Assesses readiness for USMLE Step 1 and advancement to
clinical training
Question 1
A 40-year-old woman at 5 months' gestation comes to the physician for amniocentesis.
Results show a normal 46,XY karyotype of the fetus. Four months later, the newborn is
delivered. The infant is noted to have ambiguous genitalia. Which of the following is the
most likely cause?
A) Androgen insensitivity syndrome
B) 5-alpha-reductase deficiency
C) Congenital adrenal hyperplasia
D) Klinefelter syndrome
E) Turner syndrome
Correct Answer: C
Explanation: A 46,XY karyotype with ambiguous genitalia at birth suggests impaired
pg. 1
,androgen production or action. Congenital adrenal hyperplasia due to 21-hydroxylase
deficiency can cause virilization in females, but in a 46,XY male, impaired testosterone
synthesis can lead to undervirilization. The normal karyotype with female external
genitalia in a 46,XY individual suggests complete androgen insensitivity syndrome, but
ambiguous genitalia points to partial androgen insensitivity or 5-alpha-reductase
deficiency.
Question 2
Free purine and pyrimidine bases are reutilized in normal metabolism. In children with
Lesch-Nyhan syndrome who have intellectual disability, poor muscle coordination, and
self-mutilation tendencies, there is a defect in the salvage of which of the following pairs
of bases?
A) Adenine and thymine
B) Guanine and hypoxanthine
C) Guanine and uric acid
D) Uracil and cytosine
E) Xanthine and hypoxanthine
Correct Answer: B
Explanation: Lesch-Nyhan syndrome presents with intellectual disability, aggressive
behavior, self-mutilation, gout, and dystonia. The disorder is due to inactivating
mutations of hypoxanthine-guanine phosphoribosyltransferase (HGPRT), a key enzyme
in the purine salvage pathway, inherited in an X-linked recessive fashion. HGPRT
catalyzes the conversion of guanine to guanosine monophosphate and hypoxanthine to
inosine monophosphate. Patients with deficient HGPRT activity cannot salvage guanine
and hypoxanthine, developing increased levels of xanthine and uric acid. Hyperuricemia
in Lesch-Nyhan syndrome is treated with xanthine oxidase inhibitors such as
allopurinol .
Incorrect Answer Rationales:
• Adenine and thymine (Choice A) are purine and pyrimidine bases, respectively,
handled through distinct salvage pathways not commonly involved in a single
disease.
• Guanine and uric acid (Choice C) accumulation may occur, but uric acid
accumulation is secondary to hypoxanthine accumulation.
• Uracil and cytosine (Choice D) are pyrimidine nucleotides; pyrimidine salvage is
not affected by HGPRT mutations.
pg. 2
, • Defects of xanthine and hypoxanthine (Choice E) may result from HGPRT
defects; however, HGPRT dysfunction results in impaired hypoxanthine salvage
with excessive xanthine production, rather than impaired xanthine salvage .
Question 3
A 42-year-old man is struck by a motor vehicle. His only injury is a closed fracture of the
proximal tibia. Initial neurovascular examination shows no deficits. Twenty-four hours
later, he has increased leg pain and paresthesias in the dorsal space between the first
and second toes. Which of the following is the most likely diagnosis?
A) Compartment syndrome
B) Deep vein thrombosis
C) Fat embolism syndrome
D) Peroneal nerve palsy
E) Tibial nerve palsy
Correct Answer: A
Explanation: Compartment syndrome is a limb-threatening condition characterized
by increased pressure within a closed fascial compartment, compromising tissue
perfusion. Risk factors include fractures (especially tibial), crush injuries, and
reperfusion injury. Symptoms include pain out of proportion to injury, paresthesia,
pallor, pulselessness, and paralysis. Paresthesias in the first dorsal web space suggest
involvement of the deep peroneal nerve, which runs through the anterior compartment.
The diagnosis is confirmed by intracompartmental pressure monitoring.
Question 4
A 65-year-old woman has ascites. Which of the following additional findings indicates a
diagnosis of constrictive pericarditis rather than cirrhosis?
A) Edema of the lower extremities
B) Esophageal varices
C) Hypoalbuminemia
D) Hyponatremia
E) Increased jugular venous pressure
F) Splenomegaly
Correct Answer: E
Explanation: Increased jugular venous pressure (JVP) is an expected finding in
constrictive pericarditis (CP) and would not be found in patients with volume overload
secondary to cirrhosis. CP describes a pathologic state where the pericardium loses its
elasticity, impairing diastolic filling of the heart. During the normal cardiac cycle,
increased venous return to the right atrium and ventricle during inspiration leads to
pg. 3
, transient RV expansion. In CP, the rigid pericardium prevents this expansion, leading to
elevated JVP. Cirrhosis causes ascites through portal hypertension and
hypoalbuminemia, but JVP is typically normal. The differentiation between CP and
cirrhosis is critical because CP is potentially reversible with pericardiectomy .
Question 5
A 62-year-old man with a 40-pack-year smoking history presents with progressive
shortness of breath and a chronic cough. Physical examination reveals jugular venous
distention, peripheral edema, and a loud P2. Echocardiography shows right ventricular
hypertrophy. Which of the following is the most likely underlying cause?
A) Left-sided heart failure
B) Pulmonary arterial hypertension
C) Aortic stenosis
D) Mitral regurgitation
E) Pericardial constriction
Correct Answer: B
Explanation: Pulmonary arterial hypertension from chronic hypoxemia secondary to
COPD leads to cor pulmonale with right ventricular hypertrophy, jugular venous
distention, and a loud P2 .
Question 6
A 55-year-old woman with hypertension and type 2 diabetes mellitus is started on a new
medication. She returns 2 weeks later with a persistent dry cough. Her blood pressure is
well controlled. Which medication is most likely responsible?
A) Hydrochlorothiazide
B) Amlodipine
C) Lisinopril
D) Metoprolol
E) Losartan
Correct Answer: C
Explanation: ACE inhibitors (lisinopril) commonly cause a persistent dry cough due to
accumulation of bradykinin. Angiotensin receptor blockers (losartan) do not cause this
side effect .
Question 7
A 48-year-old man presents with sudden onset of severe "tearing" chest pain radiating
to his back. His blood pressure is 180/100 mmHg in the right arm and 140/80 mmHg in
pg. 4
