NBME Comprehensive Basic Science Examination (CBSE) (USMLE Step 1) Exam –
National Board of Medical Examiners (NBME) – 2026/2027 Edition – Complete
Study Guide, Comprehensive Practice Questions, Detailed Answer Explanations,
High-Yield Basic Science Review, Integrated Organ Systems Coverage,
Pathology, Pharmacology, Physiology, Microbiology, Biochemistry, Anatomy,
Behavioral Science, Biostatistics, Clinical Correlations.
1. A 55-year-old man with a 30-pack-year smoking history presents with
hemoptysis and weight loss. Chest X-ray shows a right hilar mass. Biopsy reveals
small cells with scant cytoplasm and salt-and-pepper chromatin. Which tumor
marker is most associated with this malignancy?
• A) AFP
• B) hCG
• C) NSE (neuron-specific enolase)
• D) PSA
• Answer: C
Rationale: Small cell lung cancer (SCLC) is a neuroendocrine tumor. NSE and
chromogranin are markers for neuroendocrine differentiation. AFP/hCG are
for germ cell tumors, PSA for prostate.
2. A 28-year-old woman with systemic lupus erythematosus (SLE) develops
proteinuria and hematuria. Renal biopsy shows wire-loop lesions. What
immune complex is predominantly deposited?
• A) IgA
• B) IgG and C3
• C) IgM
• D) IgE
• Answer: B
Rationale: Class V (membranous) lupus nephritis shows subepithelial
, immune complexes of IgG, IgA, and C3, but "wire-loop" lesions are classic
for diffuse proliferative (Class IV) with IgG and C3 deposition.
3. A 6-month-old infant presents with failure to thrive, recurrent respiratory
infections, and chronic diarrhea. Serum electrophoresis shows markedly
decreased gamma globulins. B-cell counts are low. Which genetic defect is most
likely?
• A) Bruton's tyrosine kinase (BTK) mutation
• B) Adenosine deaminase (ADA) deficiency
• C) Dihydrorhodamine oxidase defect
• D) IL-2 receptor gamma chain mutation
• Answer: A
Rationale: X-linked agammaglobulinemia (Bruton's) is caused by BTK
mutation leading to lack of mature B cells and immunoglobulins. ADA
deficiency causes SCID with T-, B-, and NK-cell deficiency.
4. A 45-year-old man with chronic alcohol use presents with confusion, ataxia,
and ophthalmoplegia. Which vitamin deficiency is the cause?
• A) Vitamin B1 (Thiamine)
• B) Vitamin B3 (Niacin)
• C) Vitamin B6 (Pyridoxine)
• D) Vitamin B12
• Answer: A
Rationale: Wernicke-Korsakoff syndrome (confusion, ataxia,
ophthalmoplegia) is due to thiamine (B1) deficiency, common in alcoholics.
5. A 60-year-old woman with osteoarthritis is prescribed ibuprofen. She
develops acute kidney injury. Which renal structure is most affected?
, • A) Glomerulus
• B) Proximal tubule
• C) Afferent arteriole
• D) Collecting duct
• Answer: B
Rationale: NSAIDs cause acute interstitial nephritis and also affect the
proximal tubule due to inhibition of prostaglandin-mediated vasodilation,
but the direct toxic effect is on the proximal tubular cells.
6. A newborn has respiratory distress, bilious emesis, and a scaphoid abdomen.
X-ray shows a "double-bubble" sign. What is the most likely diagnosis?
• A) Pyloric stenosis
• B) Duodenal atresia
• C) Esophageal atresia
• D) Meconium ileus
• Answer: B
Rationale: Double-bubble sign on abdominal X-ray is classic for duodenal
atresia, often associated with Down syndrome.
7. A 32-year-old woman has a history of recurrent sinusitis and pneumonia.
Serum testing shows panhypogammaglobulinemia, but she has normal T-cell
function. Which of the following is the most likely diagnosis?
• A) Common variable immunodeficiency (CVID)
• B) Severe combined immunodeficiency (SCID)
• C) DiGeorge syndrome
• D) Ataxia-telangiectasia
, • Answer: A
Rationale: CVID presents in adulthood with recurrent sinopulmonary
infections, low IgG/IgA/IgM, and normal T-cell function.
8. A 50-year-old man with hypertension and diabetes presents with sudden
painless loss of vision in his right eye. Fundoscopy shows a pale retina with a
cherry-red spot at the macula. Which artery is occluded?
• A) Central retinal artery
• B) Central retinal vein
• C) Ophthalmic artery
• D) Posterior ciliary artery
• Answer: A
Rationale: Central retinal artery occlusion causes sudden painless
monocular blindness with a cherry-red macula (due to choroidal perfusion
preserving the fovea).
9. A 70-year-old man presents with resting tremor, rigidity, and shuffling gait.
His symptoms improve with levodopa. Which neurotransmitter is deficient?
• A) Acetylcholine
• B) Dopamine
• C) GABA
• D) Serotonin
• Answer: B
Rationale: Parkinson's disease is due to loss of dopaminergic neurons in the
substantia nigra pars compacta.
National Board of Medical Examiners (NBME) – 2026/2027 Edition – Complete
Study Guide, Comprehensive Practice Questions, Detailed Answer Explanations,
High-Yield Basic Science Review, Integrated Organ Systems Coverage,
Pathology, Pharmacology, Physiology, Microbiology, Biochemistry, Anatomy,
Behavioral Science, Biostatistics, Clinical Correlations.
1. A 55-year-old man with a 30-pack-year smoking history presents with
hemoptysis and weight loss. Chest X-ray shows a right hilar mass. Biopsy reveals
small cells with scant cytoplasm and salt-and-pepper chromatin. Which tumor
marker is most associated with this malignancy?
• A) AFP
• B) hCG
• C) NSE (neuron-specific enolase)
• D) PSA
• Answer: C
Rationale: Small cell lung cancer (SCLC) is a neuroendocrine tumor. NSE and
chromogranin are markers for neuroendocrine differentiation. AFP/hCG are
for germ cell tumors, PSA for prostate.
2. A 28-year-old woman with systemic lupus erythematosus (SLE) develops
proteinuria and hematuria. Renal biopsy shows wire-loop lesions. What
immune complex is predominantly deposited?
• A) IgA
• B) IgG and C3
• C) IgM
• D) IgE
• Answer: B
Rationale: Class V (membranous) lupus nephritis shows subepithelial
, immune complexes of IgG, IgA, and C3, but "wire-loop" lesions are classic
for diffuse proliferative (Class IV) with IgG and C3 deposition.
3. A 6-month-old infant presents with failure to thrive, recurrent respiratory
infections, and chronic diarrhea. Serum electrophoresis shows markedly
decreased gamma globulins. B-cell counts are low. Which genetic defect is most
likely?
• A) Bruton's tyrosine kinase (BTK) mutation
• B) Adenosine deaminase (ADA) deficiency
• C) Dihydrorhodamine oxidase defect
• D) IL-2 receptor gamma chain mutation
• Answer: A
Rationale: X-linked agammaglobulinemia (Bruton's) is caused by BTK
mutation leading to lack of mature B cells and immunoglobulins. ADA
deficiency causes SCID with T-, B-, and NK-cell deficiency.
4. A 45-year-old man with chronic alcohol use presents with confusion, ataxia,
and ophthalmoplegia. Which vitamin deficiency is the cause?
• A) Vitamin B1 (Thiamine)
• B) Vitamin B3 (Niacin)
• C) Vitamin B6 (Pyridoxine)
• D) Vitamin B12
• Answer: A
Rationale: Wernicke-Korsakoff syndrome (confusion, ataxia,
ophthalmoplegia) is due to thiamine (B1) deficiency, common in alcoholics.
5. A 60-year-old woman with osteoarthritis is prescribed ibuprofen. She
develops acute kidney injury. Which renal structure is most affected?
, • A) Glomerulus
• B) Proximal tubule
• C) Afferent arteriole
• D) Collecting duct
• Answer: B
Rationale: NSAIDs cause acute interstitial nephritis and also affect the
proximal tubule due to inhibition of prostaglandin-mediated vasodilation,
but the direct toxic effect is on the proximal tubular cells.
6. A newborn has respiratory distress, bilious emesis, and a scaphoid abdomen.
X-ray shows a "double-bubble" sign. What is the most likely diagnosis?
• A) Pyloric stenosis
• B) Duodenal atresia
• C) Esophageal atresia
• D) Meconium ileus
• Answer: B
Rationale: Double-bubble sign on abdominal X-ray is classic for duodenal
atresia, often associated with Down syndrome.
7. A 32-year-old woman has a history of recurrent sinusitis and pneumonia.
Serum testing shows panhypogammaglobulinemia, but she has normal T-cell
function. Which of the following is the most likely diagnosis?
• A) Common variable immunodeficiency (CVID)
• B) Severe combined immunodeficiency (SCID)
• C) DiGeorge syndrome
• D) Ataxia-telangiectasia
, • Answer: A
Rationale: CVID presents in adulthood with recurrent sinopulmonary
infections, low IgG/IgA/IgM, and normal T-cell function.
8. A 50-year-old man with hypertension and diabetes presents with sudden
painless loss of vision in his right eye. Fundoscopy shows a pale retina with a
cherry-red spot at the macula. Which artery is occluded?
• A) Central retinal artery
• B) Central retinal vein
• C) Ophthalmic artery
• D) Posterior ciliary artery
• Answer: A
Rationale: Central retinal artery occlusion causes sudden painless
monocular blindness with a cherry-red macula (due to choroidal perfusion
preserving the fovea).
9. A 70-year-old man presents with resting tremor, rigidity, and shuffling gait.
His symptoms improve with levodopa. Which neurotransmitter is deficient?
• A) Acetylcholine
• B) Dopamine
• C) GABA
• D) Serotonin
• Answer: B
Rationale: Parkinson's disease is due to loss of dopaminergic neurons in the
substantia nigra pars compacta.
