NBME CBSE Comprehensive Basic Science Examination 2026/2027 | NBME
CBSE Study Guide & Exam Prep | Comprehensive Basic Science Exam Review
| USMLE Step 1 Basic Science Preparation | Biochemistry, Biostatistics,
Epidemiology, Immunology, Microbiology, Pathology, Pharmacology,
Physiology, Anatomy, Behavioral Science, Genetics, Cardiovascular,
Respiratory, Renal, Gastrointestinal, Endocrine, Reproductive, Nervous
System, Musculoskeletal, Skin, Hematology, Multisystem Disorders,
Diagnosis, Medical Knowledge & Clinical Reasoning | CBSE Practice
Questions, Clinical Vignettes & Detailed Rationales
Question 1: A 62-year-old man presents with painless hematuria and a renal
mass. Biopsy reveals a tumor composed of cells with clear cytoplasm and
abundant glycogen. Which of the following is the most likely diagnosis?
A. Papillary renal cell carcinoma
B. Clear cell renal cell carcinoma
C. Chromophobe renal cell carcinoma
D. Transitional cell carcinoma
E. Renal oncocytoma
CORRECT ANSWER: B. Clear cell renal cell carcinoma
Rationale: Clear cell renal cell carcinoma is the most common type of renal cell
carcinoma, characterized by cells with clear cytoplasm due to glycogen and lipid
content. It originates from the proximal tubule and is associated with VHL gene
mutations.
Question 2: A 45-year-old woman presents with episodic headaches,
palpitations, and diaphoresis. Plasma metaneprines are elevated. Which of the
following is the most likely diagnosis?
A. Cushing syndrome
B. Pheochromocytoma
C. Primary aldosteronism
D. Hyperthyroidism
E. Carcinoid syndrome
CORRECT ANSWER: B. Pheochromocytoma
,Rationale: Pheochromocytoma is a tumor of the adrenal medulla that secretes
catecholamines, causing episodic hypertension, palpitations, and diaphoresis.
Elevated plasma metanephrines are highly specific for diagnosis.
Question 3: A 3-day-old neonate presents with lethargy, vomiting, and
hyperammonemia. Which of the following enzyme deficiencies is most likely?
A. Ornithine transcarbamylase
B. Fumarase
C. Pyruvate carboxylase
D. Glucose-6-phosphatase
E. Branched-chain ketoacid dehydrogenase
CORRECT ANSWER: A. Ornithine transcarbamylase
Rationale: Ornithine transcarbamylase deficiency is an X-linked urea cycle
disorder presenting in neonates with hyperammonemia, respiratory alkalosis, and
low BUN. Prompt treatment is essential to prevent neurological damage.
Question 4: A 58-year-old man with a 30-pack-year smoking history presents
with a 3-cm pulmonary mass. Biopsy reveals malignant cells with scant
cytoplasm and "salt-and-pepper" chromatin. Which of the following is the most
likely diagnosis?
A. Adenocarcinoma
B. Squamous cell carcinoma
C. Small cell carcinoma
D. Large cell carcinoma
E. Carcinoid tumor
CORRECT ANSWER: C. Small cell carcinoma
Rationale: Small cell carcinoma is strongly associated with smoking and
characterized by small cells with scant cytoplasm, neuroendocrine granules, and
"salt-and-pepper" chromatin. It is highly aggressive and often metastatic at
diagnosis.
Question 5: A 28-year-old woman presents with fatigue, weight gain, and cold
intolerance. Laboratory studies show elevated TSH and low free T4. Which of
the following is the most likely cause?
,A. Graves disease
B. Hashimoto thyroiditis
C. Subacute thyroiditis
D. Toxic multinodular goiter
E. Pituitary adenoma
CORRECT ANSWER: B. Hashimoto thyroiditis
Rationale: Hashimoto thyroiditis is an autoimmune hypothyroidism characterized
by lymphocytic infiltration and anti-thyroid peroxidase antibodies. It presents
with elevated TSH and low free T4.
Question 6: A 6-month-old infant presents with developmental delay, coarse
facial features, and hepatosplenomegaly. Enzyme assay reveals deficiency of
alpha-L-iduronidase. Which substance accumulates?
A. Glucocerebroside
B. Sphingomyelin
C. Dermatan sulfate and heparan sulfate
D. Galactocerebroside
E. Ganglioside GM2
CORRECT ANSWER: C. Dermatan sulfate and heparan sulfate
Rationale: Alpha-L-iduronidase deficiency causes Hurler syndrome
(mucopolysaccharidosis I), leading to accumulation of dermatan sulfate and
heparan sulfate in lysosomes. Clinical features include coarse facial features and
developmental delay.
Question 7: A 45-year-old man with chronic alcohol use presents with painful,
swollen joints. Synovial fluid shows needle-shaped, negatively birefringent
crystals. Which pathway is impaired?
A. Heme synthesis
B. Purine salvage
C. Pyrimidine catabolism
D. Urea cycle
E. Fatty acid oxidation
CORRECT ANSWER: B. Purine salvage
, Rationale: Needle-shaped, negatively birefringent crystals indicate gout due to
monosodium urate deposition. Chronic alcohol use impairs purine salvage,
leading to hyperuricemia and gout.
Question 8: A 24-year-old medical student is studying the biochemistry of
fasting. After 12 hours of fasting, which process is most increased in the liver?
A. Glycolysis
B. Glycogenesis
C. Glycogenolysis
D. Pentose phosphate pathway
E. Fatty acid synthesis
CORRECT ANSWER: C. Glycogenolysis
Rationale: After 12 hours of fasting, hepatic glycogen stores are mobilized via
glycogenolysis to maintain euglycemia. Glycolysis and glycogenesis are reduced
during fasting.
Question 9: A 67-year-old man undergoes total hip replacement. On
postoperative day 4, he develops dyspnea and hypoxia. V/Q scan shows
multiple segmental perfusion defects. Which mediator is responsible for acute
hemodynamic effects?
A. Interleukin-6
B. Thromboxane A2 and platelet-activating factor
C. Tumor necrosis factor-alpha
D. Complement C5a
E. Histamine
CORRECT ANSWER: B. Thromboxane A2 and platelet-activating factor
Rationale: Acute pulmonary embolism causes hemodynamic compromise via
platelet activation and release of thromboxane A2 and PAF, causing pulmonary
vasoconstriction and bronchoconstriction.
Question 10: A 19-year-old college student is evaluated for recurrent
sinopulmonary infections with encapsulated organisms. Flow cytometry shows
absent CD19+ B cells. Which of the following is the most likely diagnosis?
CBSE Study Guide & Exam Prep | Comprehensive Basic Science Exam Review
| USMLE Step 1 Basic Science Preparation | Biochemistry, Biostatistics,
Epidemiology, Immunology, Microbiology, Pathology, Pharmacology,
Physiology, Anatomy, Behavioral Science, Genetics, Cardiovascular,
Respiratory, Renal, Gastrointestinal, Endocrine, Reproductive, Nervous
System, Musculoskeletal, Skin, Hematology, Multisystem Disorders,
Diagnosis, Medical Knowledge & Clinical Reasoning | CBSE Practice
Questions, Clinical Vignettes & Detailed Rationales
Question 1: A 62-year-old man presents with painless hematuria and a renal
mass. Biopsy reveals a tumor composed of cells with clear cytoplasm and
abundant glycogen. Which of the following is the most likely diagnosis?
A. Papillary renal cell carcinoma
B. Clear cell renal cell carcinoma
C. Chromophobe renal cell carcinoma
D. Transitional cell carcinoma
E. Renal oncocytoma
CORRECT ANSWER: B. Clear cell renal cell carcinoma
Rationale: Clear cell renal cell carcinoma is the most common type of renal cell
carcinoma, characterized by cells with clear cytoplasm due to glycogen and lipid
content. It originates from the proximal tubule and is associated with VHL gene
mutations.
Question 2: A 45-year-old woman presents with episodic headaches,
palpitations, and diaphoresis. Plasma metaneprines are elevated. Which of the
following is the most likely diagnosis?
A. Cushing syndrome
B. Pheochromocytoma
C. Primary aldosteronism
D. Hyperthyroidism
E. Carcinoid syndrome
CORRECT ANSWER: B. Pheochromocytoma
,Rationale: Pheochromocytoma is a tumor of the adrenal medulla that secretes
catecholamines, causing episodic hypertension, palpitations, and diaphoresis.
Elevated plasma metanephrines are highly specific for diagnosis.
Question 3: A 3-day-old neonate presents with lethargy, vomiting, and
hyperammonemia. Which of the following enzyme deficiencies is most likely?
A. Ornithine transcarbamylase
B. Fumarase
C. Pyruvate carboxylase
D. Glucose-6-phosphatase
E. Branched-chain ketoacid dehydrogenase
CORRECT ANSWER: A. Ornithine transcarbamylase
Rationale: Ornithine transcarbamylase deficiency is an X-linked urea cycle
disorder presenting in neonates with hyperammonemia, respiratory alkalosis, and
low BUN. Prompt treatment is essential to prevent neurological damage.
Question 4: A 58-year-old man with a 30-pack-year smoking history presents
with a 3-cm pulmonary mass. Biopsy reveals malignant cells with scant
cytoplasm and "salt-and-pepper" chromatin. Which of the following is the most
likely diagnosis?
A. Adenocarcinoma
B. Squamous cell carcinoma
C. Small cell carcinoma
D. Large cell carcinoma
E. Carcinoid tumor
CORRECT ANSWER: C. Small cell carcinoma
Rationale: Small cell carcinoma is strongly associated with smoking and
characterized by small cells with scant cytoplasm, neuroendocrine granules, and
"salt-and-pepper" chromatin. It is highly aggressive and often metastatic at
diagnosis.
Question 5: A 28-year-old woman presents with fatigue, weight gain, and cold
intolerance. Laboratory studies show elevated TSH and low free T4. Which of
the following is the most likely cause?
,A. Graves disease
B. Hashimoto thyroiditis
C. Subacute thyroiditis
D. Toxic multinodular goiter
E. Pituitary adenoma
CORRECT ANSWER: B. Hashimoto thyroiditis
Rationale: Hashimoto thyroiditis is an autoimmune hypothyroidism characterized
by lymphocytic infiltration and anti-thyroid peroxidase antibodies. It presents
with elevated TSH and low free T4.
Question 6: A 6-month-old infant presents with developmental delay, coarse
facial features, and hepatosplenomegaly. Enzyme assay reveals deficiency of
alpha-L-iduronidase. Which substance accumulates?
A. Glucocerebroside
B. Sphingomyelin
C. Dermatan sulfate and heparan sulfate
D. Galactocerebroside
E. Ganglioside GM2
CORRECT ANSWER: C. Dermatan sulfate and heparan sulfate
Rationale: Alpha-L-iduronidase deficiency causes Hurler syndrome
(mucopolysaccharidosis I), leading to accumulation of dermatan sulfate and
heparan sulfate in lysosomes. Clinical features include coarse facial features and
developmental delay.
Question 7: A 45-year-old man with chronic alcohol use presents with painful,
swollen joints. Synovial fluid shows needle-shaped, negatively birefringent
crystals. Which pathway is impaired?
A. Heme synthesis
B. Purine salvage
C. Pyrimidine catabolism
D. Urea cycle
E. Fatty acid oxidation
CORRECT ANSWER: B. Purine salvage
, Rationale: Needle-shaped, negatively birefringent crystals indicate gout due to
monosodium urate deposition. Chronic alcohol use impairs purine salvage,
leading to hyperuricemia and gout.
Question 8: A 24-year-old medical student is studying the biochemistry of
fasting. After 12 hours of fasting, which process is most increased in the liver?
A. Glycolysis
B. Glycogenesis
C. Glycogenolysis
D. Pentose phosphate pathway
E. Fatty acid synthesis
CORRECT ANSWER: C. Glycogenolysis
Rationale: After 12 hours of fasting, hepatic glycogen stores are mobilized via
glycogenolysis to maintain euglycemia. Glycolysis and glycogenesis are reduced
during fasting.
Question 9: A 67-year-old man undergoes total hip replacement. On
postoperative day 4, he develops dyspnea and hypoxia. V/Q scan shows
multiple segmental perfusion defects. Which mediator is responsible for acute
hemodynamic effects?
A. Interleukin-6
B. Thromboxane A2 and platelet-activating factor
C. Tumor necrosis factor-alpha
D. Complement C5a
E. Histamine
CORRECT ANSWER: B. Thromboxane A2 and platelet-activating factor
Rationale: Acute pulmonary embolism causes hemodynamic compromise via
platelet activation and release of thromboxane A2 and PAF, causing pulmonary
vasoconstriction and bronchoconstriction.
Question 10: A 19-year-old college student is evaluated for recurrent
sinopulmonary infections with encapsulated organisms. Flow cytometry shows
absent CD19+ B cells. Which of the following is the most likely diagnosis?
