NBME CBSE COMPREHENSIVE BASIC SCIENCE
EXAMINATION 2026/2027 | Updated Edition |
Complete Solutions | USMLE Step Prep | Pass
Guaranteed - A+ Graded
BLOCK 1: Questions 1-50
Time Limit: 60 minutes
CARDIOVASCULAR SYSTEM
Q1: A 58-year-old man presents with crushing substernal chest pain radiating to his left
arm. ECG shows ST-segment elevation in leads II, III, and aVF. Cardiac catheterization
reveals 90% occlusion of the right coronary artery. Which metabolic change occurs
FIRST in irreversibly injured myocardial cells?
A. Loss of membrane phospholipids due to phospholipase activation
B. Mitochondrial swelling with loss of cristae and amorphous densities
C. Depletion of glycogen stores and cessation of aerobic glycolysis [CORRECT]
D. Nuclear chromatin clumping and pyknosis
Correct Answer: C
Rationale: This patient presents with acute inferior wall STEMI (RCA territory). The
question tests understanding of the sequence of ischemic cell injury. Irreversible
ischemic injury begins with loss of oxidative phosphorylation within seconds, forcing
,cells to rely on anaerobic glycolysis. Glycogen depletion occurs within 30-60 minutes as
cells attempt to maintain ATP through anaerobic metabolism. While mitochondrial
swelling (B) and membrane phospholipid loss (A) occur, these are later ultrastructural
changes seen after 1-2 hours. Nuclear changes (D) represent very late findings. The
EARLIEST biochemical event in irreversible injury is the shift from aerobic to failing
anaerobic metabolism with glycogen depletion. Test-taking strategy: NBME emphasizes
temporal sequences in pathology—remember "glycogen gone first" for irreversible
ischemia. Cross-reference: Review stages of cell injury (reversible vs. irreversible) and
myocardial energy metabolism.
Q2: A 45-year-old woman with a history of rheumatic heart disease presents with
dyspnea on exertion and orthopnea. Physical examination reveals an opening snap
followed by a diastolic rumble at the apex. Echocardiography shows a "hockey stick"
appearance of the anterior mitral leaflet. Which hemodynamic parameter is DECREASED
in this condition?
A. Left atrial pressure
B. Left ventricular end-diastolic volume [CORRECT]
C. Pulmonary capillary wedge pressure
D. Left atrial volume
Correct Answer: B
Rationale: This patient has mitral stenosis (opening snap, diastolic rumble, hockey stick
sign on echo = rheumatic mitral stenosis). The pathophysiology involves obstruction to
left ventricular inflow. The narrowed mitral valve impedes blood flow from left atrium to
left ventricle during diastole. Consequently, LV filling is impaired, leading to decreased
,left ventricular end-diastolic volume (preload). Left atrial pressure (A) and volume (D)
INCREASE due to backup of blood. Pulmonary capillary wedge pressure (C), which
reflects left atrial pressure, also increases, potentially causing pulmonary edema.
Distractor analysis: Students often confuse mitral stenosis with
regurgitation—remember stenosis = obstruction to inflow = decreased LV preload;
regurgitation = volume overload = increased LV preload. Test-taking insight: In valvular
lesions, trace the direction of blood flow obstruction to determine pressure/volume
changes. Cross-reference: Cardiac pressure-volume loops and valvular hemodynamics.
Q3: A 67-year-old man with atrial fibrillation is started on warfarin. Two weeks later, he
develops skin necrosis on his buttocks and thighs. Laboratory studies show protein C
activity of 35% (normal 70-140%). Which vitamin K-dependent coagulation factor has
the SHORTEST half-life, explaining this paradoxical complication?
A. Factor II (prothrombin)
B. Factor VII
C. Protein C [CORRECT]
D. Factor X
Correct Answer: C
Rationale: This patient has warfarin-induced skin necrosis, a rare complication caused
by protein C deficiency. Warfarin inhibits vitamin K epoxide reductase, depleting
functional vitamin K-dependent factors (II, VII, IX, X, Protein C, Protein S). Protein C has
the shortest half-life (6-8 hours) compared to Factor VII (6 hours, but circulates in higher
concentration), Factor II (60 hours), and Factor X (40 hours). Early in warfarin therapy,
Protein C depletion creates a transient hypercoagulable state before other factors
, decline, causing microvascular thrombosis and skin necrosis. Distractor trap: Factor VII
has a similar short half-life but its deficiency causes bleeding, not thrombosis. The key
is recognizing Protein C's anticoagulant function. Test-taking strategy: Remember "C
comes before S, and both are SHORT"—Protein C and S have shortest half-lives among
vitamin K-dependent factors. Cross-reference: Coagulation cascade, protein C/S
anticoagulant pathway, warfarin pharmacodynamics.
Q4: A 3-day-old newborn becomes cyanotic during feeding. Physical examination
reveals a harsh holosystolic murmur at the left lower sternal border. Echocardiography
shows a large defect in the muscular interventricular septum with left-to-right shunting.
Which physiologic change explains the delayed cyanosis in this condition?
A. Decreased pulmonary vascular resistance at birth
B. Increased pulmonary vascular resistance causing reversal of shunt flow [CORRECT]
C. Hypertrophy of the right ventricular outflow tract
D. Closure of the ductus arteriosus
Correct Answer: B
Rationale: This infant has a ventricular septal defect (VSD) with Eisenmenger physiology
development. Initially, high left ventricular pressure drives left-to-right shunting
(acyanotic). Over time, chronic left-to-right shunting increases pulmonary blood flow,
causing pulmonary vascular remodeling and elevated pulmonary vascular resistance
(PVR). When PVR exceeds systemic vascular resistance, the shunt reverses to
right-to-left (Eisenmenger syndrome), causing late-onset cyanosis. This explains the
"delayed cyanosis" pattern characteristic of VSD, ASD, and PDA. Distractor analysis: (A)
Decreased PVR at birth actually promotes left-to-right shunting initially. (C) RVOT
EXAMINATION 2026/2027 | Updated Edition |
Complete Solutions | USMLE Step Prep | Pass
Guaranteed - A+ Graded
BLOCK 1: Questions 1-50
Time Limit: 60 minutes
CARDIOVASCULAR SYSTEM
Q1: A 58-year-old man presents with crushing substernal chest pain radiating to his left
arm. ECG shows ST-segment elevation in leads II, III, and aVF. Cardiac catheterization
reveals 90% occlusion of the right coronary artery. Which metabolic change occurs
FIRST in irreversibly injured myocardial cells?
A. Loss of membrane phospholipids due to phospholipase activation
B. Mitochondrial swelling with loss of cristae and amorphous densities
C. Depletion of glycogen stores and cessation of aerobic glycolysis [CORRECT]
D. Nuclear chromatin clumping and pyknosis
Correct Answer: C
Rationale: This patient presents with acute inferior wall STEMI (RCA territory). The
question tests understanding of the sequence of ischemic cell injury. Irreversible
ischemic injury begins with loss of oxidative phosphorylation within seconds, forcing
,cells to rely on anaerobic glycolysis. Glycogen depletion occurs within 30-60 minutes as
cells attempt to maintain ATP through anaerobic metabolism. While mitochondrial
swelling (B) and membrane phospholipid loss (A) occur, these are later ultrastructural
changes seen after 1-2 hours. Nuclear changes (D) represent very late findings. The
EARLIEST biochemical event in irreversible injury is the shift from aerobic to failing
anaerobic metabolism with glycogen depletion. Test-taking strategy: NBME emphasizes
temporal sequences in pathology—remember "glycogen gone first" for irreversible
ischemia. Cross-reference: Review stages of cell injury (reversible vs. irreversible) and
myocardial energy metabolism.
Q2: A 45-year-old woman with a history of rheumatic heart disease presents with
dyspnea on exertion and orthopnea. Physical examination reveals an opening snap
followed by a diastolic rumble at the apex. Echocardiography shows a "hockey stick"
appearance of the anterior mitral leaflet. Which hemodynamic parameter is DECREASED
in this condition?
A. Left atrial pressure
B. Left ventricular end-diastolic volume [CORRECT]
C. Pulmonary capillary wedge pressure
D. Left atrial volume
Correct Answer: B
Rationale: This patient has mitral stenosis (opening snap, diastolic rumble, hockey stick
sign on echo = rheumatic mitral stenosis). The pathophysiology involves obstruction to
left ventricular inflow. The narrowed mitral valve impedes blood flow from left atrium to
left ventricle during diastole. Consequently, LV filling is impaired, leading to decreased
,left ventricular end-diastolic volume (preload). Left atrial pressure (A) and volume (D)
INCREASE due to backup of blood. Pulmonary capillary wedge pressure (C), which
reflects left atrial pressure, also increases, potentially causing pulmonary edema.
Distractor analysis: Students often confuse mitral stenosis with
regurgitation—remember stenosis = obstruction to inflow = decreased LV preload;
regurgitation = volume overload = increased LV preload. Test-taking insight: In valvular
lesions, trace the direction of blood flow obstruction to determine pressure/volume
changes. Cross-reference: Cardiac pressure-volume loops and valvular hemodynamics.
Q3: A 67-year-old man with atrial fibrillation is started on warfarin. Two weeks later, he
develops skin necrosis on his buttocks and thighs. Laboratory studies show protein C
activity of 35% (normal 70-140%). Which vitamin K-dependent coagulation factor has
the SHORTEST half-life, explaining this paradoxical complication?
A. Factor II (prothrombin)
B. Factor VII
C. Protein C [CORRECT]
D. Factor X
Correct Answer: C
Rationale: This patient has warfarin-induced skin necrosis, a rare complication caused
by protein C deficiency. Warfarin inhibits vitamin K epoxide reductase, depleting
functional vitamin K-dependent factors (II, VII, IX, X, Protein C, Protein S). Protein C has
the shortest half-life (6-8 hours) compared to Factor VII (6 hours, but circulates in higher
concentration), Factor II (60 hours), and Factor X (40 hours). Early in warfarin therapy,
Protein C depletion creates a transient hypercoagulable state before other factors
, decline, causing microvascular thrombosis and skin necrosis. Distractor trap: Factor VII
has a similar short half-life but its deficiency causes bleeding, not thrombosis. The key
is recognizing Protein C's anticoagulant function. Test-taking strategy: Remember "C
comes before S, and both are SHORT"—Protein C and S have shortest half-lives among
vitamin K-dependent factors. Cross-reference: Coagulation cascade, protein C/S
anticoagulant pathway, warfarin pharmacodynamics.
Q4: A 3-day-old newborn becomes cyanotic during feeding. Physical examination
reveals a harsh holosystolic murmur at the left lower sternal border. Echocardiography
shows a large defect in the muscular interventricular septum with left-to-right shunting.
Which physiologic change explains the delayed cyanosis in this condition?
A. Decreased pulmonary vascular resistance at birth
B. Increased pulmonary vascular resistance causing reversal of shunt flow [CORRECT]
C. Hypertrophy of the right ventricular outflow tract
D. Closure of the ductus arteriosus
Correct Answer: B
Rationale: This infant has a ventricular septal defect (VSD) with Eisenmenger physiology
development. Initially, high left ventricular pressure drives left-to-right shunting
(acyanotic). Over time, chronic left-to-right shunting increases pulmonary blood flow,
causing pulmonary vascular remodeling and elevated pulmonary vascular resistance
(PVR). When PVR exceeds systemic vascular resistance, the shunt reverses to
right-to-left (Eisenmenger syndrome), causing late-onset cyanosis. This explains the
"delayed cyanosis" pattern characteristic of VSD, ASD, and PDA. Distractor analysis: (A)
Decreased PVR at birth actually promotes left-to-right shunting initially. (C) RVOT
