NBME CBSE ACTUAL TEST QUESTIONS AND
ANSWERS 2026/2027 (QUIZ BANK WITH ALL
THE CORRECT ANSWERS) (USMLE STEP 1)
MEDICAL EXAMINATION
This comprehensive practice examination is meticulously designed for candidates preparing for
the NBME Comprehensive Basic Science Examination (CBSE) and the USMLE Step 1. Aligned
with the 2026/2027 curriculum, this document serves as a definitive study guide and exam
review, featuring 100 questions that mirror the complexity and rigor of the actual certification
examination. It covers both theoretical foundations and clinical applications, including anatomy,
biochemistry, microbiology, pathology, pharmacology, physiology, behavioral sciences, and
biostatistics. Each question is accompanied by a detailed, verified solution to ensure 100%
correct answers and a deep, integrated understanding of basic science principles, guaranteeing
distinction-level preparation for this critical medical credential.
TABLE OF CONTENTS
1. Biochemistry, Molecular Biology, and Genetics (Questions 1-15)
2. Cardiovascular Physiology and Pathology (Questions 16-25)
3. Pulmonary and Renal Physiology (Questions 26-35)
4. Gastrointestinal and Hepatobiliary Systems (Questions 36-40)
5. Endocrine and Reproductive Systems (Questions 41-50)
6. Neurology and Musculoskeletal Systems (Questions 51-60)
7. Microbiology and Immunology (Questions 61-70)
8. Pharmacology and Toxicology (Questions 71-80)
9. Pathology and Neoplasia (Questions 81-90)
10. Behavioral Sciences, Biostatistics, and Ethics (Questions 91-100)
,SECTION 1: BIOCHEMISTRY, MOLECULAR BIOLOGY, AND GENETICS
Question 1
A 4-day-old boy is brought to the emergency department with lethargy, poor feeding, and
jaundice. Laboratory studies show hypoglycemia, elevated blood lactate, and increased urinary
4-hydroxyphenylacetate. Skin fibroblast enzyme assay demonstrates deficient activity of
dihydroperidine reductase (DHPR). Which of the following best explains why dietary
phenylalanine restriction alone will NOT prevent neurologic deterioration in this patient?
A) Tyrosine becomes an essential amino acid
B) Dopamine and serotonin synthesis are impaired in the brain
C) Branched-chain amino acids accumulate systemically
D) Tetrahydrobiopterin is overproduced in liver mitochondria
E) Gluconeogenesis is irreversibly blocked in hepatocytes
Correct Answer: B
DHPR deficiency causes a rare "malignant" form of phenylketonuria in which
tetrahydrobiopterin (BH4), the obligate cofactor for phenylalanine hydroxylase, cannot be
regenerated. Consequently BH4 is also unavailable to brain tyrosine and tryptophan
hydroxylases, so dopamine and serotonin synthesis collapse despite dietary phenylalanine
restriction, leading to irreversible neurotransmitter deficiency and neurologic decline.
Question 2
A 25-year-old woman with a strong family history of early myocardial infarction is found to
have an LDL-cholesterol of 410 mg/dL. Molecular analysis shows homozygosity for a missense
mutation in exon 4 of the LDL-receptor gene that encodes the ligand-binding domain. Which of
the following intracellular events is most directly impaired in hepatocytes of this patient?
A) Clathrin-mediated endocytosis of apoB-100-containing particles
B) Lysosomal degradation of cholesteryl esters
C) Release of free cholesterol from late endosomes
D) Proteasomal degradation of HMG-CoA reductase
E) SREBP-2 activation in the Golgi
Correct Answer: A
The described mutation produces classic homozygous familial hypercholesterolemia. A defective
ligand-binding domain prevents the LDL receptor from clustering into clathrin-coated pits, so
hepatocytes cannot perform receptor-mediated endocytosis of LDL particles. Lysosomal
hydrolysis and NPC1-mediated cholesterol release occur normally once internalization happens.
HMG-CoA reductase degradation and SREBP-2 processing are downstream of intracellular
cholesterol sensing and are actually up-regulated because LDL-derived cholesterol delivery is
low.
,Question 3
A 6-year-old girl develops acute hemolytic anemia after eating fava beans. Peripheral-blood
smear shows Heinz bodies and bite cells. Glucose-6-phosphate dehydrogenase (G6PD) activity is
5% of normal. Which metabolic intermediate accumulates and drives oxidative damage to
hemoglobin in this patient?
A) Glucose-6-phosphate
B) 6-Phosphogluconate
C) Ribulose-5-phosphate
D) NADP+
E) Hydrogen peroxide
Correct Answer: E
G6PD deficiency impairs the pentose-phosphate pathway, decreasing NADPH production. Low
NADPH prevents glutathione reductase from maintaining reduced glutathione (GSH), so
hydrogen peroxide and other reactive oxygen species accumulate and cause oxidative damage to
hemoglobin, forming Heinz bodies.
Question 4
A 4-year-old boy presents with intellectual disability and self-mutilating behavior. Lab results
show hyperuricemia. Which enzyme is deficient?
A) HGPRT
B) Adenosine deaminase
C) Purine nucleoside phosphorylase
D) Xanthine oxidase
Correct Answer: A
Lesch-Nyhan syndrome is caused by a deficiency in hypoxanthine-guanine
phosphoribosyltransferase (HGPRT), leading to excess uric acid and CNS symptoms including
self-mutilation, intellectual disability, and choreoathetosis.
Question 5
A patient with a history of 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 (Cobalamin)
Correct Answer: A
Wernicke-Korsakoff syndrome results from thiamine deficiency, often seen in chronic alcoholism
due to poor nutrition and impaired absorption. The classic triad is confusion, ataxia, and
ophthalmoplegia.
, Question 6
Which enzyme is the rate-limiting step in glycolysis?
A) Hexokinase
B) Phosphofructokinase-1 (PFK-1)
C) Aldolase
D) Pyruvate kinase
Correct Answer: B
PFK-1 is the primary regulatory enzyme of glycolysis, inhibited by ATP and citrate and
activated by AMP and fructose-2,6-bisphosphate.
Question 7
A patient presents with painful muscle cramps during exercise. Lab tests show elevated
myoglobin and failure of blood lactate to rise. Which disease is suspected?
A) McArdle disease
B) Pompe disease
C) Cori disease
D) Von Gierke disease
Correct Answer: A
McArdle disease (Type V glycogen storage disease) is a deficiency of skeletal muscle glycogen
phosphorylase, preventing the breakdown of glycogen during exertion. This leads to muscle
cramps, myoglobinuria, and failure of blood lactate to rise with exercise.
Question 8
Which amino acid is the precursor for serotonin?
A) Tryptophan
B) Tyrosine
C) Phenylalanine
D) Histidine
Correct Answer: A
Tryptophan is hydroxylated and decarboxylated to form serotonin. It is also the precursor for
melatonin and niacin.
Question 9
A 20-year-old female presents with fair skin, blue eyes, and a 'mousy' body odor. What is the
most likely diagnosis?
A) Phenylketonuria (PKU)
B) Alkaptonuria
ANSWERS 2026/2027 (QUIZ BANK WITH ALL
THE CORRECT ANSWERS) (USMLE STEP 1)
MEDICAL EXAMINATION
This comprehensive practice examination is meticulously designed for candidates preparing for
the NBME Comprehensive Basic Science Examination (CBSE) and the USMLE Step 1. Aligned
with the 2026/2027 curriculum, this document serves as a definitive study guide and exam
review, featuring 100 questions that mirror the complexity and rigor of the actual certification
examination. It covers both theoretical foundations and clinical applications, including anatomy,
biochemistry, microbiology, pathology, pharmacology, physiology, behavioral sciences, and
biostatistics. Each question is accompanied by a detailed, verified solution to ensure 100%
correct answers and a deep, integrated understanding of basic science principles, guaranteeing
distinction-level preparation for this critical medical credential.
TABLE OF CONTENTS
1. Biochemistry, Molecular Biology, and Genetics (Questions 1-15)
2. Cardiovascular Physiology and Pathology (Questions 16-25)
3. Pulmonary and Renal Physiology (Questions 26-35)
4. Gastrointestinal and Hepatobiliary Systems (Questions 36-40)
5. Endocrine and Reproductive Systems (Questions 41-50)
6. Neurology and Musculoskeletal Systems (Questions 51-60)
7. Microbiology and Immunology (Questions 61-70)
8. Pharmacology and Toxicology (Questions 71-80)
9. Pathology and Neoplasia (Questions 81-90)
10. Behavioral Sciences, Biostatistics, and Ethics (Questions 91-100)
,SECTION 1: BIOCHEMISTRY, MOLECULAR BIOLOGY, AND GENETICS
Question 1
A 4-day-old boy is brought to the emergency department with lethargy, poor feeding, and
jaundice. Laboratory studies show hypoglycemia, elevated blood lactate, and increased urinary
4-hydroxyphenylacetate. Skin fibroblast enzyme assay demonstrates deficient activity of
dihydroperidine reductase (DHPR). Which of the following best explains why dietary
phenylalanine restriction alone will NOT prevent neurologic deterioration in this patient?
A) Tyrosine becomes an essential amino acid
B) Dopamine and serotonin synthesis are impaired in the brain
C) Branched-chain amino acids accumulate systemically
D) Tetrahydrobiopterin is overproduced in liver mitochondria
E) Gluconeogenesis is irreversibly blocked in hepatocytes
Correct Answer: B
DHPR deficiency causes a rare "malignant" form of phenylketonuria in which
tetrahydrobiopterin (BH4), the obligate cofactor for phenylalanine hydroxylase, cannot be
regenerated. Consequently BH4 is also unavailable to brain tyrosine and tryptophan
hydroxylases, so dopamine and serotonin synthesis collapse despite dietary phenylalanine
restriction, leading to irreversible neurotransmitter deficiency and neurologic decline.
Question 2
A 25-year-old woman with a strong family history of early myocardial infarction is found to
have an LDL-cholesterol of 410 mg/dL. Molecular analysis shows homozygosity for a missense
mutation in exon 4 of the LDL-receptor gene that encodes the ligand-binding domain. Which of
the following intracellular events is most directly impaired in hepatocytes of this patient?
A) Clathrin-mediated endocytosis of apoB-100-containing particles
B) Lysosomal degradation of cholesteryl esters
C) Release of free cholesterol from late endosomes
D) Proteasomal degradation of HMG-CoA reductase
E) SREBP-2 activation in the Golgi
Correct Answer: A
The described mutation produces classic homozygous familial hypercholesterolemia. A defective
ligand-binding domain prevents the LDL receptor from clustering into clathrin-coated pits, so
hepatocytes cannot perform receptor-mediated endocytosis of LDL particles. Lysosomal
hydrolysis and NPC1-mediated cholesterol release occur normally once internalization happens.
HMG-CoA reductase degradation and SREBP-2 processing are downstream of intracellular
cholesterol sensing and are actually up-regulated because LDL-derived cholesterol delivery is
low.
,Question 3
A 6-year-old girl develops acute hemolytic anemia after eating fava beans. Peripheral-blood
smear shows Heinz bodies and bite cells. Glucose-6-phosphate dehydrogenase (G6PD) activity is
5% of normal. Which metabolic intermediate accumulates and drives oxidative damage to
hemoglobin in this patient?
A) Glucose-6-phosphate
B) 6-Phosphogluconate
C) Ribulose-5-phosphate
D) NADP+
E) Hydrogen peroxide
Correct Answer: E
G6PD deficiency impairs the pentose-phosphate pathway, decreasing NADPH production. Low
NADPH prevents glutathione reductase from maintaining reduced glutathione (GSH), so
hydrogen peroxide and other reactive oxygen species accumulate and cause oxidative damage to
hemoglobin, forming Heinz bodies.
Question 4
A 4-year-old boy presents with intellectual disability and self-mutilating behavior. Lab results
show hyperuricemia. Which enzyme is deficient?
A) HGPRT
B) Adenosine deaminase
C) Purine nucleoside phosphorylase
D) Xanthine oxidase
Correct Answer: A
Lesch-Nyhan syndrome is caused by a deficiency in hypoxanthine-guanine
phosphoribosyltransferase (HGPRT), leading to excess uric acid and CNS symptoms including
self-mutilation, intellectual disability, and choreoathetosis.
Question 5
A patient with a history of 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 (Cobalamin)
Correct Answer: A
Wernicke-Korsakoff syndrome results from thiamine deficiency, often seen in chronic alcoholism
due to poor nutrition and impaired absorption. The classic triad is confusion, ataxia, and
ophthalmoplegia.
, Question 6
Which enzyme is the rate-limiting step in glycolysis?
A) Hexokinase
B) Phosphofructokinase-1 (PFK-1)
C) Aldolase
D) Pyruvate kinase
Correct Answer: B
PFK-1 is the primary regulatory enzyme of glycolysis, inhibited by ATP and citrate and
activated by AMP and fructose-2,6-bisphosphate.
Question 7
A patient presents with painful muscle cramps during exercise. Lab tests show elevated
myoglobin and failure of blood lactate to rise. Which disease is suspected?
A) McArdle disease
B) Pompe disease
C) Cori disease
D) Von Gierke disease
Correct Answer: A
McArdle disease (Type V glycogen storage disease) is a deficiency of skeletal muscle glycogen
phosphorylase, preventing the breakdown of glycogen during exertion. This leads to muscle
cramps, myoglobinuria, and failure of blood lactate to rise with exercise.
Question 8
Which amino acid is the precursor for serotonin?
A) Tryptophan
B) Tyrosine
C) Phenylalanine
D) Histidine
Correct Answer: A
Tryptophan is hydroxylated and decarboxylated to form serotonin. It is also the precursor for
melatonin and niacin.
Question 9
A 20-year-old female presents with fair skin, blue eyes, and a 'mousy' body odor. What is the
most likely diagnosis?
A) Phenylketonuria (PKU)
B) Alkaptonuria
