NBME CBSE 2026/2027 | Comprehensive Basic Science Examination
Study Guide, NBME CBSE Exam Prep, USMLE Step 1 Review, Medical
School Basic Sciences, Pathology, Physiology, Pharmacology,
Microbiology, Biochemistry, Anatomy, Immunology, Biostatistics,
Practice Questions & Explanations
Question 1: A 24-year-old medical student is studying the biochemistry of
fasting. After 12 hours of fasting, which of the following processes is most
increased in the liver to maintain blood glucose levels?
A. Glycolysis
B. Glycogenesis
C. Glycogenolysis
D. Pentose phosphate pathway
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,
and the pentose phosphate pathway primarily generates NADPH and ribose-5-
phosphate, not glucose.
Question 2: A 3-day-old neonate presents with poor feeding, vomiting, and
lethargy. Labs show hyperammonemia, respiratory alkalosis, and low BUN.
Which of the following enzyme deficiencies is most likely?
A. Ornithine transcarbamylase
B. Fumarase
C. Pyruvate carboxylase
D. Glucose-6-phosphatase
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. Fumarase and pyruvate carboxylase defects do not primarily cause
hyperammonemia; glucose-6-phosphatase deficiency causes von Gierke disease
with hypoglycemia and lactic acidosis.
Question 3: A 45-year-old man with chronic alcohol use presents with painful,
swollen joints and a history of recurrent kidney stones. Synovial fluid analysis
,shows needle-shaped, negatively birefringent crystals. Which of the following
metabolic pathways is most likely impaired?
A. Heme synthesis
B. Purine salvage
C. Pyrimidine catabolism
D. Urea cycle
CORRECT ANSWER: B. Purine salvage
Rationale: Needle-shaped, negatively birefringent crystals indicate gout due to
monosodium urate deposition. Chronic alcohol use and kidney stones suggest
hyperuricemia from impaired purine salvage, particularly decreased
hypoxanthine-guanine phosphoribosyltransferase (HGPRT) activity or increased
purine degradation.
Question 4: A 6-month-old infant presents with developmental delay, coarse
facial features, and hepatosplenomegaly. Enzyme assay reveals deficiency of
alpha-L-iduronidase. Which of the following substances accumulates in
lysosomes?
A. Glucocerebroside
B. Sphingomyelin
C. Dermatan sulfate and heparan sulfate
D. Galactocerebroside
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. Glucocerebroside accumulates in Gaucher disease,
sphingomyelin in Niemann-Pick disease, and galactocerebroside in Krabbe
disease.
Question 5: A 32-year-old woman presents with fatigue, weight gain, cold
intolerance, and constipation. Labs show low free T4, elevated TSH, and
elevated anti-thyroid peroxidase antibodies. Which of the following is the most
likely diagnosis?
,A. Graves disease
B. Hashimoto thyroiditis
C. Subacute thyroiditis
D. Toxic multinodular goiter
CORRECT ANSWER: B. Hashimoto thyroiditis
Rationale: Hashimoto thyroiditis is an autoimmune hypothyroidism characterized
by low free T4, elevated TSH, and positive anti-thyroid peroxidase antibodies.
Graves disease causes hyperthyroidism; subacute thyroiditis often has a painful
thyroid and transient thyrotoxicosis; toxic multinodular goiter causes
hyperthyroidism.
Question 6: A 58-year-old man with a 40-pack-year smoking history presents
with hemoptysis and weight loss. Chest X-ray shows a hilar mass. Biopsy reveals
small blue cells with scant cytoplasm and neuroendocrine markers. Which of the
following paraneoplastic syndromes is most associated with this tumor?
A. Hypercalcemia
B. Syndrome of inappropriate ADH secretion
C. Cushing syndrome from ectopic ACTH
D. Lambert-Eaton myasthenic syndrome
CORRECT ANSWER: B. Syndrome of inappropriate ADH secretion
Rationale: Small cell lung carcinoma is a neuroendocrine tumor often associated
with SIADH and ectopic ACTH. Hypercalcemia is more typical of squamous cell
carcinoma; Lambert-Eaton syndrome is associated with small cell lung cancer but
SIADH is more common. The question asks most associated; SIADH is a classic
paraneoplastic syndrome.
Question 7: A 7-year-old boy is brought to the clinic with recurrent
sinopulmonary infections, poor growth, and clubbing of the fingers. Sweat
chloride test is elevated. Which of the following is the most likely underlying
defect?
A. Defect in CFTR chloride channel
B. Defect in alpha-1 antitrypsin
, C. Defect in dystrophin
D. Defect in fibrillin
CORRECT ANSWER: A. Defect in CFTR chloride channel
Rationale: Elevated sweat chloride and recurrent sinopulmonary infections with
clubbing suggest cystic fibrosis, caused by mutations in the CFTR chloride channel.
Alpha-1 antitrypsin deficiency causes liver and lung disease; dystrophin defects
cause muscular dystrophy; fibrillin defects cause Marfan syndrome.
Question 8: A 62-year-old man presents with sudden onset of severe tearing
chest pain radiating to the back. He has a history of hypertension. CT
angiography shows an intimal tear in the ascending aorta. Which of the
following is the most appropriate immediate management?
A. Intravenous beta-blocker and surgical consultation
B. Intravenous thrombolytics
C. Aspirin and observation
D. Calcium channel blocker and discharge
CORRECT ANSWER: A. Intravenous beta-blocker and surgical consultation
Rationale: Stanford type A aortic dissection involves the ascending aorta and is a
surgical emergency. Immediate management includes blood pressure control with
intravenous beta-blockers to reduce shear stress, followed by urgent surgical
repair. Thrombolytics are contraindicated.
Question 9: A 28-year-old woman presents with palpitations, heat intolerance,
and weight loss. Physical exam reveals a diffuse goiter and exophthalmos. Labs
show low TSH, elevated free T4, and positive TSH receptor antibodies. Which of
the following is the most likely diagnosis?
A. Hashimoto thyroiditis
B. Graves disease
C. Toxic adenoma
D. Thyroid carcinoma
CORRECT ANSWER: B. Graves disease
Study Guide, NBME CBSE Exam Prep, USMLE Step 1 Review, Medical
School Basic Sciences, Pathology, Physiology, Pharmacology,
Microbiology, Biochemistry, Anatomy, Immunology, Biostatistics,
Practice Questions & Explanations
Question 1: A 24-year-old medical student is studying the biochemistry of
fasting. After 12 hours of fasting, which of the following processes is most
increased in the liver to maintain blood glucose levels?
A. Glycolysis
B. Glycogenesis
C. Glycogenolysis
D. Pentose phosphate pathway
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,
and the pentose phosphate pathway primarily generates NADPH and ribose-5-
phosphate, not glucose.
Question 2: A 3-day-old neonate presents with poor feeding, vomiting, and
lethargy. Labs show hyperammonemia, respiratory alkalosis, and low BUN.
Which of the following enzyme deficiencies is most likely?
A. Ornithine transcarbamylase
B. Fumarase
C. Pyruvate carboxylase
D. Glucose-6-phosphatase
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. Fumarase and pyruvate carboxylase defects do not primarily cause
hyperammonemia; glucose-6-phosphatase deficiency causes von Gierke disease
with hypoglycemia and lactic acidosis.
Question 3: A 45-year-old man with chronic alcohol use presents with painful,
swollen joints and a history of recurrent kidney stones. Synovial fluid analysis
,shows needle-shaped, negatively birefringent crystals. Which of the following
metabolic pathways is most likely impaired?
A. Heme synthesis
B. Purine salvage
C. Pyrimidine catabolism
D. Urea cycle
CORRECT ANSWER: B. Purine salvage
Rationale: Needle-shaped, negatively birefringent crystals indicate gout due to
monosodium urate deposition. Chronic alcohol use and kidney stones suggest
hyperuricemia from impaired purine salvage, particularly decreased
hypoxanthine-guanine phosphoribosyltransferase (HGPRT) activity or increased
purine degradation.
Question 4: A 6-month-old infant presents with developmental delay, coarse
facial features, and hepatosplenomegaly. Enzyme assay reveals deficiency of
alpha-L-iduronidase. Which of the following substances accumulates in
lysosomes?
A. Glucocerebroside
B. Sphingomyelin
C. Dermatan sulfate and heparan sulfate
D. Galactocerebroside
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. Glucocerebroside accumulates in Gaucher disease,
sphingomyelin in Niemann-Pick disease, and galactocerebroside in Krabbe
disease.
Question 5: A 32-year-old woman presents with fatigue, weight gain, cold
intolerance, and constipation. Labs show low free T4, elevated TSH, and
elevated anti-thyroid peroxidase antibodies. Which of the following is the most
likely diagnosis?
,A. Graves disease
B. Hashimoto thyroiditis
C. Subacute thyroiditis
D. Toxic multinodular goiter
CORRECT ANSWER: B. Hashimoto thyroiditis
Rationale: Hashimoto thyroiditis is an autoimmune hypothyroidism characterized
by low free T4, elevated TSH, and positive anti-thyroid peroxidase antibodies.
Graves disease causes hyperthyroidism; subacute thyroiditis often has a painful
thyroid and transient thyrotoxicosis; toxic multinodular goiter causes
hyperthyroidism.
Question 6: A 58-year-old man with a 40-pack-year smoking history presents
with hemoptysis and weight loss. Chest X-ray shows a hilar mass. Biopsy reveals
small blue cells with scant cytoplasm and neuroendocrine markers. Which of the
following paraneoplastic syndromes is most associated with this tumor?
A. Hypercalcemia
B. Syndrome of inappropriate ADH secretion
C. Cushing syndrome from ectopic ACTH
D. Lambert-Eaton myasthenic syndrome
CORRECT ANSWER: B. Syndrome of inappropriate ADH secretion
Rationale: Small cell lung carcinoma is a neuroendocrine tumor often associated
with SIADH and ectopic ACTH. Hypercalcemia is more typical of squamous cell
carcinoma; Lambert-Eaton syndrome is associated with small cell lung cancer but
SIADH is more common. The question asks most associated; SIADH is a classic
paraneoplastic syndrome.
Question 7: A 7-year-old boy is brought to the clinic with recurrent
sinopulmonary infections, poor growth, and clubbing of the fingers. Sweat
chloride test is elevated. Which of the following is the most likely underlying
defect?
A. Defect in CFTR chloride channel
B. Defect in alpha-1 antitrypsin
, C. Defect in dystrophin
D. Defect in fibrillin
CORRECT ANSWER: A. Defect in CFTR chloride channel
Rationale: Elevated sweat chloride and recurrent sinopulmonary infections with
clubbing suggest cystic fibrosis, caused by mutations in the CFTR chloride channel.
Alpha-1 antitrypsin deficiency causes liver and lung disease; dystrophin defects
cause muscular dystrophy; fibrillin defects cause Marfan syndrome.
Question 8: A 62-year-old man presents with sudden onset of severe tearing
chest pain radiating to the back. He has a history of hypertension. CT
angiography shows an intimal tear in the ascending aorta. Which of the
following is the most appropriate immediate management?
A. Intravenous beta-blocker and surgical consultation
B. Intravenous thrombolytics
C. Aspirin and observation
D. Calcium channel blocker and discharge
CORRECT ANSWER: A. Intravenous beta-blocker and surgical consultation
Rationale: Stanford type A aortic dissection involves the ascending aorta and is a
surgical emergency. Immediate management includes blood pressure control with
intravenous beta-blockers to reduce shear stress, followed by urgent surgical
repair. Thrombolytics are contraindicated.
Question 9: A 28-year-old woman presents with palpitations, heat intolerance,
and weight loss. Physical exam reveals a diffuse goiter and exophthalmos. Labs
show low TSH, elevated free T4, and positive TSH receptor antibodies. Which of
the following is the most likely diagnosis?
A. Hashimoto thyroiditis
B. Graves disease
C. Toxic adenoma
D. Thyroid carcinoma
CORRECT ANSWER: B. Graves disease
