NBME CBSE Actual Test Questions and Answers
(2026/2027) | USMLE STEP 1 PREPARATION
QUIZ BANK
National Board of Medical Examiners (NBME) Comprehensive Basic Science Examination
(CBSE) | Core Domains: Anatomy, Behavioral Sciences, Biochemistry & Nutrition,
Microbiology & Immunology, Pathology, Pharmacology, Physiology, and Genetics &
Molecular Biology | USMLE Step 1 Readiness Focus | Standardized Medical Licensing
Assessment Format
Exam Structure
The NBME CBSE for the 2026/2027 academic cycle is a 200-question, multiple-choice question
(MCQ) examination, simulating the length, content, and difficulty of the USMLE Step 1.
Introduction
This NBME CBSE quiz bank for the 2026/2027 cycle provides actual-style questions to prepare
medical students for this critical benchmark exam. The content reflects the integrated,
clinical-vignette style of the USMLE Step 1, testing application of basic science knowledge to
clinical scenarios and assessing readiness for the national licensing examination.
Answer Format
All correct answers and key biomedical concepts must be presented in bold and green,
followed by detailed rationales that explain the underlying basic science principle, integrate
knowledge across disciplines, and clarify the clinical reasoning required to answer the
vignette-based question correctly.
Question 1: A 28-year-old man presents with fatigue, pallor, and shortness of breath on
exertion. Laboratory studies show hemoglobin of 7.2 g/dL, mean corpuscular volume (MCV) of
110 fL, and elevated serum homocysteine with normal methylmalonic acid levels. Which of the
following is the most likely diagnosis?
(A) Iron deficiency anemia
(B) Vitamin B12 deficiency
(C) Folate deficiency
(D) Anemia of chronic disease
,(E) Sideroblastic anemia
Correct Answer: (C) Folate deficiency
Rationale: This patient has macrocytic anemia (MCV > 100 fL) with elevated homocysteine but
normal methylmalonic acid. Both folate and vitamin B12 are required for homocysteine
metabolism, but only B12 is a cofactor for methylmalonyl-CoA mutase. Thus, B12 deficiency
causes elevation of both homocysteine and methylmalonic acid, whereas folate deficiency
elevates homocysteine alone. Iron deficiency and anemia of chronic disease cause microcytic or
normocytic anemia, not macrocytic. Sideroblastic anemia typically presents with ringed
sideroblasts and may be microcytic or dimorphic.
Question 2: A 55-year-old woman with a history of hypertension and type 2 diabetes mellitus
presents with sudden-onset severe headache, vomiting, and loss of consciousness. CT scan of the
head reveals a hemorrhage in the basal ganglia. Which of the following arteries is most likely
ruptured?
(A) Anterior cerebral artery
(B) Middle cerebral artery
(C) Posterior cerebral artery
(D) Lenticulostriate arteries
(E) Basilar artery
Correct Answer: (D) Lenticulostriate arteries
Rationale: Hypertensive intracerebral hemorrhages most commonly occur in the basal ganglia,
thalamus, pons, or cerebellum. The lenticulostriate arteries are small penetrating branches of
the middle cerebral artery that supply the basal ganglia and internal capsule. Chronic
hypertension causes lipohyalinosis and microaneurysm formation (Charcot-Bouchard
aneurysms) in these small vessels, predisposing them to rupture. The other listed arteries are
larger and less commonly involved in primary hypertensive hemorrhage.
Question 3: A 3-year-old boy is brought to the physician because of recurrent bacterial
infections, including pneumonia and otitis media. Laboratory studies show markedly decreased
,levels of immunoglobulins of all classes. Flow cytometry reveals a near absence of CD19+ B cells.
Which of the following is the most likely diagnosis?
(A) DiGeorge syndrome
(B) Wiskott-Aldrich syndrome
(C) X-linked agammaglobulinemia
(D) Severe combined immunodeficiency (SCID)
(E) Chronic granulomatous disease
Correct Answer: (C) X-linked agammaglobulinemia
Rationale: X-linked agammaglobulinemia (Bruton’s disease) results from a mutation in the
BTK (Bruton tyrosine kinase) gene, which is essential for B-cell maturation. Patients present
after 6 months of age (when maternal antibodies wane) with recurrent pyogenic infections and
profoundly low levels of all immunoglobulin classes. Flow cytometry shows absent or very low
CD19+ B cells, while T-cell numbers are normal. DiGeorge involves T-cell deficiency due to
thymic aplasia. SCID affects both T and B cells. Wiskott-Aldrich includes thrombocytopenia and
eczema. Chronic granulomatous disease involves defective phagocyte oxidative burst, not
antibody deficiency.
Question 4: A 62-year-old man with a 40-pack-year smoking history presents with weight loss,
hemoptysis, and hypercalcemia. Chest X-ray shows a right hilar mass. Which of the following
substances is most likely secreted by the tumor?
(A) Adrenocorticotropic hormone (ACTH)
(B) Antidiuretic hormone (ADH)
(C) Parathyroid hormone-related protein (PTHrP)
, (D) Serotonin
(E) Calcitonin
Correct Answer: (C) Parathyroid hormone-related protein (PTHrP)
Rationale: Squamous cell carcinoma of the lung is strongly associated with smoking and
commonly causes paraneoplastic hypercalcemia via secretion of PTHrP, which mimics the
action of parathyroid hormone, leading to increased bone resorption and renal calcium
reabsorption. Small cell carcinoma is more commonly associated with ADH (SIADH) or ACTH
secretion. Carcinoid tumors secrete serotonin. Medullary thyroid carcinoma produces
calcitonin. This clinical picture—smoking history, hilar mass, hypercalcemia—is classic for
squamous cell carcinoma with PTHrP-mediated hypercalcemia.
Question 5: A 24-year-old woman presents with fever, sore throat, cervical lymphadenopathy,
and atypical lymphocytosis. Peripheral blood smear shows large lymphocytes with abundant
cytoplasm and vacuoles. Heterophile antibody test is positive. Which of the following viruses is
the most likely cause?
(A) Cytomegalovirus (CMV)
(B) Epstein-Barr virus (EBV)
(C) Human herpesvirus 8 (HHV-8)
(D) Herpes simplex virus type 1 (HSV-1)
(E) Varicella-zoster virus (VZV)
Correct Answer: (B) Epstein-Barr virus (EBV)
Rationale: Infectious mononucleosis caused by EBV classically presents with fever,
pharyngitis, lymphadenopathy, and atypical lymphocytosis (activated CD8+ T cells reacting to
EBV-infected B cells). The heterophile antibody test (Monospot) is positive in most cases. CMV
can cause a mononucleosis-like illness but is heterophile-negative. HHV-8 is associated with
Kaposi sarcoma. HSV-1 causes oral or genital ulcers, and VZV causes chickenpox or shingles.
(2026/2027) | USMLE STEP 1 PREPARATION
QUIZ BANK
National Board of Medical Examiners (NBME) Comprehensive Basic Science Examination
(CBSE) | Core Domains: Anatomy, Behavioral Sciences, Biochemistry & Nutrition,
Microbiology & Immunology, Pathology, Pharmacology, Physiology, and Genetics &
Molecular Biology | USMLE Step 1 Readiness Focus | Standardized Medical Licensing
Assessment Format
Exam Structure
The NBME CBSE for the 2026/2027 academic cycle is a 200-question, multiple-choice question
(MCQ) examination, simulating the length, content, and difficulty of the USMLE Step 1.
Introduction
This NBME CBSE quiz bank for the 2026/2027 cycle provides actual-style questions to prepare
medical students for this critical benchmark exam. The content reflects the integrated,
clinical-vignette style of the USMLE Step 1, testing application of basic science knowledge to
clinical scenarios and assessing readiness for the national licensing examination.
Answer Format
All correct answers and key biomedical concepts must be presented in bold and green,
followed by detailed rationales that explain the underlying basic science principle, integrate
knowledge across disciplines, and clarify the clinical reasoning required to answer the
vignette-based question correctly.
Question 1: A 28-year-old man presents with fatigue, pallor, and shortness of breath on
exertion. Laboratory studies show hemoglobin of 7.2 g/dL, mean corpuscular volume (MCV) of
110 fL, and elevated serum homocysteine with normal methylmalonic acid levels. Which of the
following is the most likely diagnosis?
(A) Iron deficiency anemia
(B) Vitamin B12 deficiency
(C) Folate deficiency
(D) Anemia of chronic disease
,(E) Sideroblastic anemia
Correct Answer: (C) Folate deficiency
Rationale: This patient has macrocytic anemia (MCV > 100 fL) with elevated homocysteine but
normal methylmalonic acid. Both folate and vitamin B12 are required for homocysteine
metabolism, but only B12 is a cofactor for methylmalonyl-CoA mutase. Thus, B12 deficiency
causes elevation of both homocysteine and methylmalonic acid, whereas folate deficiency
elevates homocysteine alone. Iron deficiency and anemia of chronic disease cause microcytic or
normocytic anemia, not macrocytic. Sideroblastic anemia typically presents with ringed
sideroblasts and may be microcytic or dimorphic.
Question 2: A 55-year-old woman with a history of hypertension and type 2 diabetes mellitus
presents with sudden-onset severe headache, vomiting, and loss of consciousness. CT scan of the
head reveals a hemorrhage in the basal ganglia. Which of the following arteries is most likely
ruptured?
(A) Anterior cerebral artery
(B) Middle cerebral artery
(C) Posterior cerebral artery
(D) Lenticulostriate arteries
(E) Basilar artery
Correct Answer: (D) Lenticulostriate arteries
Rationale: Hypertensive intracerebral hemorrhages most commonly occur in the basal ganglia,
thalamus, pons, or cerebellum. The lenticulostriate arteries are small penetrating branches of
the middle cerebral artery that supply the basal ganglia and internal capsule. Chronic
hypertension causes lipohyalinosis and microaneurysm formation (Charcot-Bouchard
aneurysms) in these small vessels, predisposing them to rupture. The other listed arteries are
larger and less commonly involved in primary hypertensive hemorrhage.
Question 3: A 3-year-old boy is brought to the physician because of recurrent bacterial
infections, including pneumonia and otitis media. Laboratory studies show markedly decreased
,levels of immunoglobulins of all classes. Flow cytometry reveals a near absence of CD19+ B cells.
Which of the following is the most likely diagnosis?
(A) DiGeorge syndrome
(B) Wiskott-Aldrich syndrome
(C) X-linked agammaglobulinemia
(D) Severe combined immunodeficiency (SCID)
(E) Chronic granulomatous disease
Correct Answer: (C) X-linked agammaglobulinemia
Rationale: X-linked agammaglobulinemia (Bruton’s disease) results from a mutation in the
BTK (Bruton tyrosine kinase) gene, which is essential for B-cell maturation. Patients present
after 6 months of age (when maternal antibodies wane) with recurrent pyogenic infections and
profoundly low levels of all immunoglobulin classes. Flow cytometry shows absent or very low
CD19+ B cells, while T-cell numbers are normal. DiGeorge involves T-cell deficiency due to
thymic aplasia. SCID affects both T and B cells. Wiskott-Aldrich includes thrombocytopenia and
eczema. Chronic granulomatous disease involves defective phagocyte oxidative burst, not
antibody deficiency.
Question 4: A 62-year-old man with a 40-pack-year smoking history presents with weight loss,
hemoptysis, and hypercalcemia. Chest X-ray shows a right hilar mass. Which of the following
substances is most likely secreted by the tumor?
(A) Adrenocorticotropic hormone (ACTH)
(B) Antidiuretic hormone (ADH)
(C) Parathyroid hormone-related protein (PTHrP)
, (D) Serotonin
(E) Calcitonin
Correct Answer: (C) Parathyroid hormone-related protein (PTHrP)
Rationale: Squamous cell carcinoma of the lung is strongly associated with smoking and
commonly causes paraneoplastic hypercalcemia via secretion of PTHrP, which mimics the
action of parathyroid hormone, leading to increased bone resorption and renal calcium
reabsorption. Small cell carcinoma is more commonly associated with ADH (SIADH) or ACTH
secretion. Carcinoid tumors secrete serotonin. Medullary thyroid carcinoma produces
calcitonin. This clinical picture—smoking history, hilar mass, hypercalcemia—is classic for
squamous cell carcinoma with PTHrP-mediated hypercalcemia.
Question 5: A 24-year-old woman presents with fever, sore throat, cervical lymphadenopathy,
and atypical lymphocytosis. Peripheral blood smear shows large lymphocytes with abundant
cytoplasm and vacuoles. Heterophile antibody test is positive. Which of the following viruses is
the most likely cause?
(A) Cytomegalovirus (CMV)
(B) Epstein-Barr virus (EBV)
(C) Human herpesvirus 8 (HHV-8)
(D) Herpes simplex virus type 1 (HSV-1)
(E) Varicella-zoster virus (VZV)
Correct Answer: (B) Epstein-Barr virus (EBV)
Rationale: Infectious mononucleosis caused by EBV classically presents with fever,
pharyngitis, lymphadenopathy, and atypical lymphocytosis (activated CD8+ T cells reacting to
EBV-infected B cells). The heterophile antibody test (Monospot) is positive in most cases. CMV
can cause a mononucleosis-like illness but is heterophile-negative. HHV-8 is associated with
Kaposi sarcoma. HSV-1 causes oral or genital ulcers, and VZV causes chickenpox or shingles.
