NBME CBSE | Comprehensive Basic Science
Examination Study Guide & Exam Prep 2026/2027
| NBME CBSE Medical School Review, USMLE Step
1 Preparation, Pathology, Pharmacology,
Physiology, Microbiology & Immunology,
Biochemistry, Anatomy, Behavioral Science,
Biostatistics, Organ-System Integration, Clinical
Diagnosis, Medical Knowledge, Practice Questions,
Answers & Detailed Explanations
Question 1: A 22-year-old male presents with a two-day history of severe,
colicky abdominal pain, nausea, and vomiting. He has a history of two prior
episodes of similar pain that resolved spontaneously. An abdominal CT scan
reveals a thickened ileocecal valve and proximal dilation of the small bowel.
Which of the following pathological processes is most likely responsible for his
condition?
A. Fibrous adhesions from prior surgery
B. Intussusception of the terminal ileum
C. Congenital malrotation of the midgut
D. A neoplastic polyp acting as a lead point
CORRECT ANSWER: B. Intussusception of the terminal ileum
Rationale: The patient's presentation of intermittent colicky abdominal pain with a CT
showing a thickened ileocecal valve and proximal bowel dilation is classic for
intussusception in an adult. In adults, intussusception is often due to a lead point, which
can be a neoplastic polyp, but the description of a thickened ileocecal valve itself acting
as the lead point is the most common cause of ileocolic intussusception. While fibrous
adhesions and malrotation can cause obstruction, they do not typically present with the
"target sign" or the specific anatomical finding of a thickened valve acting as the lead
point. A neoplastic polyp is a possible lead point, but the question specifies the process
responsible for the condition, which is the invagination of the bowel (intussusception),
often with the ileocecal valve as the lead point in this demographic.
Question 2: A 45-year-old woman with a history of hypertension and type 2
diabetes mellitus presents to the emergency department with a sudden onset
of severe, "tearing" chest pain that radiates to her back. Her blood pressure is
180/110 mmHg in the right arm and 100/70 mmHg in the left arm. A chest x-
ray shows a widened mediastinum. Which of the following is the most likely
diagnosis?
A. Acute myocardial infarction
B. Pulmonary embolism
,C. Aortic dissection
D. Esophageal rupture
CORRECT ANSWER: C. Aortic dissection
Rationale: The classic presentation of an acute aortic dissection includes sudden, severe
"tearing" or "ripping" chest pain that radiates to the back, often associated with a
difference in blood pressure between arms. The patient's history of hypertension is a
major risk factor. A widened mediastinum on chest x-ray is a supportive finding. An
acute myocardial infarction (A) typically presents with pressure-like pain and EKG
changes. A pulmonary embolism (B) often presents with sudden dyspnea and pleuritic
pain. Esophageal rupture (D) (Boerhaave syndrome) usually follows vomiting and
presents with chest pain and subcutaneous emphysema.
Question 3: A 68-year-old male is brought to the emergency department after
collapsing at home. He is confused and has a fever of 39.0°C. His family
reports that he has been complaining of a headache and neck stiffness for the
past two days. A lumbar puncture is performed, and the cerebrospinal fluid
(CSF) is turbid and has an elevated opening pressure. Gram stain of the CSF
reveals gram-positive diplococci. Which of the following virulence factors is
most directly responsible for this organism's ability to cause disease by
evading phagocytosis?
A. Lipopolysaccharide (LPS) endotoxin
B. Polysaccharide capsule
C. Pili
D. Exotoxin A
CORRECT ANSWER: B. Polysaccharide capsule
Rationale: The patient's presentation of fever, headache, neck stiffness, and turbid CSF
with gram-positive diplococci is classic for bacterial meningitis caused by Streptococcus
pneumoniae. The primary virulence factor of S. pneumoniae is its polysaccharide capsule,
which is antiphagocytic. The capsule allows the bacteria to evade the host's immune
system by preventing opsonization and phagocytosis by neutrophils and macrophages.
Lipopolysaccharide (A) is a component of gram-negative bacteria. Pili (C) are for
adherence. Exotoxin A (D) is produced by Pseudomonas aeruginosa.
Question 4: A 55-year-old man with a 30-pack-year smoking history presents
with a persistent cough, hemoptysis, and weight loss. A chest CT reveals a 4-
cm mass in the right upper lobe with mediastinal lymphadenopathy. A biopsy
of the mass is performed, and histological examination reveals small, round,
blue cells with scant cytoplasm and a high nuclear-to-cytoplasmic ratio.
Immunohistochemical staining is positive for cytokeratin and neural cell
,adhesion molecule (NCAM). Which of the following is the most likely
diagnosis?
A. Squamous cell carcinoma
B. Adenocarcinoma
C. Small cell lung cancer
D. Large cell carcinoma
CORRECT ANSWER: C. Small cell lung cancer
Rationale: The description of small, round, blue cells with scant cytoplasm is
pathognomonic for small cell lung cancer (SCLC). These tumors are highly aggressive,
strongly associated with smoking, and often present with a central mass and mediastinal
lymphadenopathy. The immunohistochemical staining positive for cytokeratin
(epithelial) and NCAM (neuroendocrine) confirms the neuroendocrine origin of SCLC.
Squamous cell carcinoma (A) shows keratinization and intercellular bridges.
Adenocarcinoma (B) shows glandular differentiation. Large cell carcinoma (D) is a
diagnosis of exclusion for non-small cell carcinomas without squamous or glandular
differentiation.
Question 5: A 32-year-old woman presents with fatigue, weight gain, and cold
intolerance. Laboratory studies reveal a TSH of 15 µIU/mL (normal 0.4-4.0)
and a free T4 of 0.5 ng/dL (normal 0.8-1.8). Thyroid peroxidase antibodies are
found to be positive. Which of the following is the most likely underlying
mechanism of her disease?
A. Granulomatous inflammation of the thyroid gland
B. Autoimmune destruction of the thyroid gland
C. Pituitary adenoma secreting TSH
D. Resistance to thyroid hormone at the target tissue
CORRECT ANSWER: B. Autoimmune destruction of the thyroid gland
Rationale: The patient has elevated TSH and low free T4, indicating primary
hypothyroidism. The presence of positive thyroid peroxidase (TPO) antibodies is highly
specific for Hashimoto's thyroiditis, an autoimmune disorder where the body's immune
system attacks and destroys the thyroid gland. Granulomatous inflammation (A) is seen
in subacute (de Quervain's) thyroiditis, which typically follows a viral infection and
presents with a painful goiter. A pituitary adenoma secreting TSH (C) would cause
elevated TSH and elevated T4/T3. Resistance to thyroid hormone (D) would cause
elevated TSH with high or normal T4/T3 levels.
Question 6: A 47-year-old man with chronic alcoholism is brought to the
emergency department with severe upper abdominal pain that radiates to his
back. He is vomiting and appears acutely ill. His serum amylase is 950 U/L
, (normal <130 U/L) and lipase is 1,200 U/L (normal <60 U/L). An abdominal
CT shows an enlarged, edematous pancreas with peripancreatic fluid
collections. Which of the following is the most common pathogenetic
mechanism for this condition in this patient population?
A. Obstruction of the pancreatic duct by a gallstone
B. Direct toxic effect of alcohol or its metabolites on acinar cells
C. Autoimmune destruction of pancreatic islet cells
D. Hypercalcemia leading to intraductal precipitation
CORRECT ANSWER: B. Direct toxic effect of alcohol or its metabolites on
acinar cells
Rationale: The patient has acute pancreatitis, evidenced by severe abdominal pain
radiating to the back and markedly elevated serum amylase and lipase. In patients with
chronic alcoholism, the most common mechanism is the direct toxic effect of alcohol or
its metabolites (e.g., fatty acid ethyl esters) on pancreatic acinar cells. This leads to the
premature activation of trypsinogen to trypsin and autodigestion of the pancreas.
Obstruction by a gallstone (A) is the most common cause of acute pancreatitis overall,
but not in the setting of chronic alcoholism. Autoimmune destruction (C) is associated
with autoimmune pancreatitis. Hypercalcemia (D) is a less common cause and can lead
to intraductal calculi.
Question 7: A 25-year-old man presents with a history of recurrent epistaxis
(nosebleeds) and a family history of similar problems. On examination, he has
multiple small, red-to-purple papules on his lips, oral mucosa, and fingertips.
He also has a history of gastrointestinal bleeding. Which of the following is the
most likely diagnosis?
A. Osler-Weber-Rendu syndrome
B. Ehlers-Danlos syndrome
C. Marfan syndrome
D. Sturge-Weber syndrome
CORRECT ANSWER: A. Osler-Weber-Rendu syndrome
Rationale: Osler-Weber-Rendu syndrome, also known as hereditary hemorrhagic
telangiectasia (HHT), is an autosomal dominant disorder characterized by
mucocutaneous telangiectasias (small, red-to-purple papules) and arteriovenous
malformations (AVMs) in internal organs, such as the lungs, brain, and GI tract.
Recurrent epistaxis is the most common presenting symptom. Ehlers-Danlos syndrome
(B) is characterized by hyperelastic skin and joint hypermobility. Marfan syndrome (C)
involves the skeletal system, eyes, and cardiovascular system (aortic root dilation).
Sturge-Weber syndrome (D) is characterized by a facial port-wine stain and
leptomeningeal angiomatosis.
Examination Study Guide & Exam Prep 2026/2027
| NBME CBSE Medical School Review, USMLE Step
1 Preparation, Pathology, Pharmacology,
Physiology, Microbiology & Immunology,
Biochemistry, Anatomy, Behavioral Science,
Biostatistics, Organ-System Integration, Clinical
Diagnosis, Medical Knowledge, Practice Questions,
Answers & Detailed Explanations
Question 1: A 22-year-old male presents with a two-day history of severe,
colicky abdominal pain, nausea, and vomiting. He has a history of two prior
episodes of similar pain that resolved spontaneously. An abdominal CT scan
reveals a thickened ileocecal valve and proximal dilation of the small bowel.
Which of the following pathological processes is most likely responsible for his
condition?
A. Fibrous adhesions from prior surgery
B. Intussusception of the terminal ileum
C. Congenital malrotation of the midgut
D. A neoplastic polyp acting as a lead point
CORRECT ANSWER: B. Intussusception of the terminal ileum
Rationale: The patient's presentation of intermittent colicky abdominal pain with a CT
showing a thickened ileocecal valve and proximal bowel dilation is classic for
intussusception in an adult. In adults, intussusception is often due to a lead point, which
can be a neoplastic polyp, but the description of a thickened ileocecal valve itself acting
as the lead point is the most common cause of ileocolic intussusception. While fibrous
adhesions and malrotation can cause obstruction, they do not typically present with the
"target sign" or the specific anatomical finding of a thickened valve acting as the lead
point. A neoplastic polyp is a possible lead point, but the question specifies the process
responsible for the condition, which is the invagination of the bowel (intussusception),
often with the ileocecal valve as the lead point in this demographic.
Question 2: A 45-year-old woman with a history of hypertension and type 2
diabetes mellitus presents to the emergency department with a sudden onset
of severe, "tearing" chest pain that radiates to her back. Her blood pressure is
180/110 mmHg in the right arm and 100/70 mmHg in the left arm. A chest x-
ray shows a widened mediastinum. Which of the following is the most likely
diagnosis?
A. Acute myocardial infarction
B. Pulmonary embolism
,C. Aortic dissection
D. Esophageal rupture
CORRECT ANSWER: C. Aortic dissection
Rationale: The classic presentation of an acute aortic dissection includes sudden, severe
"tearing" or "ripping" chest pain that radiates to the back, often associated with a
difference in blood pressure between arms. The patient's history of hypertension is a
major risk factor. A widened mediastinum on chest x-ray is a supportive finding. An
acute myocardial infarction (A) typically presents with pressure-like pain and EKG
changes. A pulmonary embolism (B) often presents with sudden dyspnea and pleuritic
pain. Esophageal rupture (D) (Boerhaave syndrome) usually follows vomiting and
presents with chest pain and subcutaneous emphysema.
Question 3: A 68-year-old male is brought to the emergency department after
collapsing at home. He is confused and has a fever of 39.0°C. His family
reports that he has been complaining of a headache and neck stiffness for the
past two days. A lumbar puncture is performed, and the cerebrospinal fluid
(CSF) is turbid and has an elevated opening pressure. Gram stain of the CSF
reveals gram-positive diplococci. Which of the following virulence factors is
most directly responsible for this organism's ability to cause disease by
evading phagocytosis?
A. Lipopolysaccharide (LPS) endotoxin
B. Polysaccharide capsule
C. Pili
D. Exotoxin A
CORRECT ANSWER: B. Polysaccharide capsule
Rationale: The patient's presentation of fever, headache, neck stiffness, and turbid CSF
with gram-positive diplococci is classic for bacterial meningitis caused by Streptococcus
pneumoniae. The primary virulence factor of S. pneumoniae is its polysaccharide capsule,
which is antiphagocytic. The capsule allows the bacteria to evade the host's immune
system by preventing opsonization and phagocytosis by neutrophils and macrophages.
Lipopolysaccharide (A) is a component of gram-negative bacteria. Pili (C) are for
adherence. Exotoxin A (D) is produced by Pseudomonas aeruginosa.
Question 4: A 55-year-old man with a 30-pack-year smoking history presents
with a persistent cough, hemoptysis, and weight loss. A chest CT reveals a 4-
cm mass in the right upper lobe with mediastinal lymphadenopathy. A biopsy
of the mass is performed, and histological examination reveals small, round,
blue cells with scant cytoplasm and a high nuclear-to-cytoplasmic ratio.
Immunohistochemical staining is positive for cytokeratin and neural cell
,adhesion molecule (NCAM). Which of the following is the most likely
diagnosis?
A. Squamous cell carcinoma
B. Adenocarcinoma
C. Small cell lung cancer
D. Large cell carcinoma
CORRECT ANSWER: C. Small cell lung cancer
Rationale: The description of small, round, blue cells with scant cytoplasm is
pathognomonic for small cell lung cancer (SCLC). These tumors are highly aggressive,
strongly associated with smoking, and often present with a central mass and mediastinal
lymphadenopathy. The immunohistochemical staining positive for cytokeratin
(epithelial) and NCAM (neuroendocrine) confirms the neuroendocrine origin of SCLC.
Squamous cell carcinoma (A) shows keratinization and intercellular bridges.
Adenocarcinoma (B) shows glandular differentiation. Large cell carcinoma (D) is a
diagnosis of exclusion for non-small cell carcinomas without squamous or glandular
differentiation.
Question 5: A 32-year-old woman presents with fatigue, weight gain, and cold
intolerance. Laboratory studies reveal a TSH of 15 µIU/mL (normal 0.4-4.0)
and a free T4 of 0.5 ng/dL (normal 0.8-1.8). Thyroid peroxidase antibodies are
found to be positive. Which of the following is the most likely underlying
mechanism of her disease?
A. Granulomatous inflammation of the thyroid gland
B. Autoimmune destruction of the thyroid gland
C. Pituitary adenoma secreting TSH
D. Resistance to thyroid hormone at the target tissue
CORRECT ANSWER: B. Autoimmune destruction of the thyroid gland
Rationale: The patient has elevated TSH and low free T4, indicating primary
hypothyroidism. The presence of positive thyroid peroxidase (TPO) antibodies is highly
specific for Hashimoto's thyroiditis, an autoimmune disorder where the body's immune
system attacks and destroys the thyroid gland. Granulomatous inflammation (A) is seen
in subacute (de Quervain's) thyroiditis, which typically follows a viral infection and
presents with a painful goiter. A pituitary adenoma secreting TSH (C) would cause
elevated TSH and elevated T4/T3. Resistance to thyroid hormone (D) would cause
elevated TSH with high or normal T4/T3 levels.
Question 6: A 47-year-old man with chronic alcoholism is brought to the
emergency department with severe upper abdominal pain that radiates to his
back. He is vomiting and appears acutely ill. His serum amylase is 950 U/L
, (normal <130 U/L) and lipase is 1,200 U/L (normal <60 U/L). An abdominal
CT shows an enlarged, edematous pancreas with peripancreatic fluid
collections. Which of the following is the most common pathogenetic
mechanism for this condition in this patient population?
A. Obstruction of the pancreatic duct by a gallstone
B. Direct toxic effect of alcohol or its metabolites on acinar cells
C. Autoimmune destruction of pancreatic islet cells
D. Hypercalcemia leading to intraductal precipitation
CORRECT ANSWER: B. Direct toxic effect of alcohol or its metabolites on
acinar cells
Rationale: The patient has acute pancreatitis, evidenced by severe abdominal pain
radiating to the back and markedly elevated serum amylase and lipase. In patients with
chronic alcoholism, the most common mechanism is the direct toxic effect of alcohol or
its metabolites (e.g., fatty acid ethyl esters) on pancreatic acinar cells. This leads to the
premature activation of trypsinogen to trypsin and autodigestion of the pancreas.
Obstruction by a gallstone (A) is the most common cause of acute pancreatitis overall,
but not in the setting of chronic alcoholism. Autoimmune destruction (C) is associated
with autoimmune pancreatitis. Hypercalcemia (D) is a less common cause and can lead
to intraductal calculi.
Question 7: A 25-year-old man presents with a history of recurrent epistaxis
(nosebleeds) and a family history of similar problems. On examination, he has
multiple small, red-to-purple papules on his lips, oral mucosa, and fingertips.
He also has a history of gastrointestinal bleeding. Which of the following is the
most likely diagnosis?
A. Osler-Weber-Rendu syndrome
B. Ehlers-Danlos syndrome
C. Marfan syndrome
D. Sturge-Weber syndrome
CORRECT ANSWER: A. Osler-Weber-Rendu syndrome
Rationale: Osler-Weber-Rendu syndrome, also known as hereditary hemorrhagic
telangiectasia (HHT), is an autosomal dominant disorder characterized by
mucocutaneous telangiectasias (small, red-to-purple papules) and arteriovenous
malformations (AVMs) in internal organs, such as the lungs, brain, and GI tract.
Recurrent epistaxis is the most common presenting symptom. Ehlers-Danlos syndrome
(B) is characterized by hyperelastic skin and joint hypermobility. Marfan syndrome (C)
involves the skeletal system, eyes, and cardiovascular system (aortic root dilation).
Sturge-Weber syndrome (D) is characterized by a facial port-wine stain and
leptomeningeal angiomatosis.
