NBME 26 questions and answers 2026\2027
A+ Grade
1
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Exam Section 1: Item 2 of 50
National, Board of Medical Examiners
Comprehensive Basic Science Self-Assessment
2. Free purine and pyrimidine bases are reutilized in normal metabolism. In children with Lesch-Nyhan
syndrome who have intellectual disability, poor muscle coordination, and self-mutilation tendencies,
there is a defect in the salvage of which of the following
pairs of bases?
A) Adenine and thymine
B) Guanine and hypoxanthine
C) Guanine and uric acid
D) Uracil and cytosine
E) Xanthine and hypoxanthine
- correct answer B.
Lesch-Nyhan syndrome presents with intellectual disability, aggressive behavior, self-mutilation, gout,
and dystonia. The disorder is due to inactivating mutations of hypoxanthine-guanine
phosphoribosyltransferase (HGPRT), a key enzyme in the purine salvage
pathway, and is inherited in an X-linked recessive fashion. HGPRT catalyzes the conversion of guanine to
guanosine monophosphate and hypoxanthine to inosine monophosphate. Patients with deficient
activity of HGPRT are unable to salvage guanine and
hypoxanthine and develop resultant increased levels of xanthine and uric acid. Hyperuricemia in Lesch-
Nyhan syndrome is treated with xanthine oxidase inhibitors, such as allopurinol or febuxostat, in order
to reduce the synthesis of uric acid.
Incorrect Answers: A, C, D and E.
,Adenine and thymine (Choice A) are purine and pyrimidine bases, respectively. Purine and pyrimidine
salvage are handled through two distinct pathways that are not commonly involved in a single disease
process.
Guanine and uric acid (Choice C) accumulation may occur as part of Lesch-Nyhan syndrome, however,
the accumulation of uric acid is also secondary to accumulation of hypoxanthine. Choice B more
accurately describes defective salvage of guanine and
hypoxanthine as the fundamental effect of HGPRT dysfunction. The accumulation of uric acid is
secondary.
Uracil and cytosine (Choice D) are pyrimidine nucleotides. Pyrimidine salvage is not affected by
mutations of HGPRT.
Defects of xanthine and hypoxanthine (Choice E) metabolism may result from defects in HGPRT.
However, HGPRT dysfunction results in impaired hypoxanthine salvage with resultant excessive
production of xanthine, rather than impaired xanthine salvage.
Educational Objective: Lesch-Nyhan syndrome presents with intellectual disability, aggressive behavior,
self-mutilation, gout, and dyst
2
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Exam Section 1: Item 3 of 50
National Board of Medical Examiners
Comprehensive Basic Science Self-Assessment
3. A 42-year-old man is struck by a motor vehicle. His only injury is a closed fracture of the proximal
tibia. Initial neurovascular examination shows no deficits. Twenty-four hours later, he has increased leg
pain and paresthesias in the dorsal space between his
first and second toes. The patient begins to pass dark red urine and becomes oliguric. Urinalysis is
positive for blood but no erythrocytes are seen on microscopic examination. Which of the following
acute disorders is the most likely cause of the renal
failure?
A) Glomerulonephritis
B) Hemolytic-uremic syndrome
C) Interstitial nephritis
D) Nephrotic syndrome
E) Tubular necrosis
- correct answer E.
,Tibial fractures present a high risk for compartment syndrome. The fracture results in blood vessel injury
and muscle injury, inflammation, and edema. Because the fascia containing the anterior compartment
of the leg does not stretch, bleeding and swelling can
cause increased pressure in the compartment. This increased pressure in turn inhibits venous drainage,
further increasing pressure in the compartment. Eventually the nerve supply and associated arteries are
compromised, leading to the classic signs and
symptoms of compartment syndrome. Signs and symptoms of compartment syndrome include pain out
of proportion to examination findings, pain with passive movement of the muscles, paresthesia, pallor,
pulselessness, and paralysis. Compromised blood supply
deprives muscle and tissue of oxygen and glucose, leading to tissue ischemia and necrosis. Muscle
necrosis leads to rhabdomyolysis, myoglobinuria, and acute renal failure. Evaluation of rhabdomyolysis
reveals red or brown urine and urinalysis is typically positive
for blood due to the presence of myoglobinuria without microscopic evidence of red blood cells. A
complication of rhabdomyolysis is acute kidney injury from acute tubular necrosis secondary to the
release of nephrotoxic myoglobin and nonprotein heme pigments.
Acute tubular necrosis typically occurs following an ischemic or nephrotoxic insult to the kidneys, which
results in loss of the tubular epithelium. Granular, muddy brown casts are common on urinalysis.
Compartment syndrome is treated by immediate fasciotomy to
decrease compartment pressure and support tissue perfusion.
Incorrect Answers: A, B, C, and D.
Glomerulonephritis (Choice A) refers to a variety of glomerular diseases, including nephritic and
nephrotic syndromes. Nephritic syndromes typically present with acute renal failure associated with h
3
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Exam Section 1: Item 5 of 50
National, Board of Medical Examiners
Comprehensive Basic Science Self-Assessment
5. A 65-year-old woman has ascites. Which of the following additional findings indicates a diagnosis of
constrictive pericarditis rather than cirrhosis?
O A) Edema of the lower extremities
B) Esophageal varices
C) Hypoalbuminemia
D) Hyponatremia
, E) Increased jugular venous pressure
F) Splenomegaly
- correct answer E.
Increased jugular venous pressure (JVP) is an expected finding in constrictive pericarditis (CP) and would
not be found in patients with volume overload secondary to cirrhosis. CP describes a pathologic state
whereby the pericardium, which encases the entirety of
the heart and the origins of the great vessels, loses its elasticity. This can occur in patients with viral
infections, connective tissue disease, tuberculosis, or as a result of cardiac surgery or radiation. During
the normal cardiac cycle, increased venous return to the
right atrium (RA) and right ventricle (RV) during inspiration leads to transient expansion of the RV with
slight bowing of the interventricular septum into the left ventricle (LV). This increased RV preload does
not impair LV filling as the pericardial sac expands to
accommodate the increased RV volume. In constrictive pericarditis, the pericardial sac loses its elasticity.
When venous return to the right heart increases, the pericardial sac is unable to expand, which
exacerbates movement of the interventricular septum into the
LV. This impairs diastolic filling of the LV and reduces cardiac output. LV diastolic filling is further
reduced by a reduction in preload from the pulmonary veins. The constricted pericardium does not
respond to normal changes in intrathoracic pressure during
inspiration, but the pulmonary venous system, which lies outside of the pericardium, experiences a
normal drop in pressure during inspiration. This difference creates an abnormal pressure gradient that
reduces LV preload and leads to reduced cardiac output. The
RV and LV develop interventricular dependence whereby increased pressure in each ventricle begins to
affect the other ventricle. CP ultimately results in equalization of pressures in all four chambers, with
clinical evidence of right heart failure exhib
4
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Exam Section 1: Item 4 of 50
National Board of Medical Examiners
Comprehensive Basic Science Self-Assessment
4. A male newborn is found to have a defect in anchoring fibrils. Which of the following skin findings is
most likely in this patient?
A) Blisters
B) Easy bruising
A+ Grade
1
----------
Exam Section 1: Item 2 of 50
National, Board of Medical Examiners
Comprehensive Basic Science Self-Assessment
2. Free purine and pyrimidine bases are reutilized in normal metabolism. In children with Lesch-Nyhan
syndrome who have intellectual disability, poor muscle coordination, and self-mutilation tendencies,
there is a defect in the salvage of which of the following
pairs of bases?
A) Adenine and thymine
B) Guanine and hypoxanthine
C) Guanine and uric acid
D) Uracil and cytosine
E) Xanthine and hypoxanthine
- correct answer B.
Lesch-Nyhan syndrome presents with intellectual disability, aggressive behavior, self-mutilation, gout,
and dystonia. The disorder is due to inactivating mutations of hypoxanthine-guanine
phosphoribosyltransferase (HGPRT), a key enzyme in the purine salvage
pathway, and is inherited in an X-linked recessive fashion. HGPRT catalyzes the conversion of guanine to
guanosine monophosphate and hypoxanthine to inosine monophosphate. Patients with deficient
activity of HGPRT are unable to salvage guanine and
hypoxanthine and develop resultant increased levels of xanthine and uric acid. Hyperuricemia in Lesch-
Nyhan syndrome is treated with xanthine oxidase inhibitors, such as allopurinol or febuxostat, in order
to reduce the synthesis of uric acid.
Incorrect Answers: A, C, D and E.
,Adenine and thymine (Choice A) are purine and pyrimidine bases, respectively. Purine and pyrimidine
salvage are handled through two distinct pathways that are not commonly involved in a single disease
process.
Guanine and uric acid (Choice C) accumulation may occur as part of Lesch-Nyhan syndrome, however,
the accumulation of uric acid is also secondary to accumulation of hypoxanthine. Choice B more
accurately describes defective salvage of guanine and
hypoxanthine as the fundamental effect of HGPRT dysfunction. The accumulation of uric acid is
secondary.
Uracil and cytosine (Choice D) are pyrimidine nucleotides. Pyrimidine salvage is not affected by
mutations of HGPRT.
Defects of xanthine and hypoxanthine (Choice E) metabolism may result from defects in HGPRT.
However, HGPRT dysfunction results in impaired hypoxanthine salvage with resultant excessive
production of xanthine, rather than impaired xanthine salvage.
Educational Objective: Lesch-Nyhan syndrome presents with intellectual disability, aggressive behavior,
self-mutilation, gout, and dyst
2
----------
Exam Section 1: Item 3 of 50
National Board of Medical Examiners
Comprehensive Basic Science Self-Assessment
3. A 42-year-old man is struck by a motor vehicle. His only injury is a closed fracture of the proximal
tibia. Initial neurovascular examination shows no deficits. Twenty-four hours later, he has increased leg
pain and paresthesias in the dorsal space between his
first and second toes. The patient begins to pass dark red urine and becomes oliguric. Urinalysis is
positive for blood but no erythrocytes are seen on microscopic examination. Which of the following
acute disorders is the most likely cause of the renal
failure?
A) Glomerulonephritis
B) Hemolytic-uremic syndrome
C) Interstitial nephritis
D) Nephrotic syndrome
E) Tubular necrosis
- correct answer E.
,Tibial fractures present a high risk for compartment syndrome. The fracture results in blood vessel injury
and muscle injury, inflammation, and edema. Because the fascia containing the anterior compartment
of the leg does not stretch, bleeding and swelling can
cause increased pressure in the compartment. This increased pressure in turn inhibits venous drainage,
further increasing pressure in the compartment. Eventually the nerve supply and associated arteries are
compromised, leading to the classic signs and
symptoms of compartment syndrome. Signs and symptoms of compartment syndrome include pain out
of proportion to examination findings, pain with passive movement of the muscles, paresthesia, pallor,
pulselessness, and paralysis. Compromised blood supply
deprives muscle and tissue of oxygen and glucose, leading to tissue ischemia and necrosis. Muscle
necrosis leads to rhabdomyolysis, myoglobinuria, and acute renal failure. Evaluation of rhabdomyolysis
reveals red or brown urine and urinalysis is typically positive
for blood due to the presence of myoglobinuria without microscopic evidence of red blood cells. A
complication of rhabdomyolysis is acute kidney injury from acute tubular necrosis secondary to the
release of nephrotoxic myoglobin and nonprotein heme pigments.
Acute tubular necrosis typically occurs following an ischemic or nephrotoxic insult to the kidneys, which
results in loss of the tubular epithelium. Granular, muddy brown casts are common on urinalysis.
Compartment syndrome is treated by immediate fasciotomy to
decrease compartment pressure and support tissue perfusion.
Incorrect Answers: A, B, C, and D.
Glomerulonephritis (Choice A) refers to a variety of glomerular diseases, including nephritic and
nephrotic syndromes. Nephritic syndromes typically present with acute renal failure associated with h
3
----------
Exam Section 1: Item 5 of 50
National, Board of Medical Examiners
Comprehensive Basic Science Self-Assessment
5. A 65-year-old woman has ascites. Which of the following additional findings indicates a diagnosis of
constrictive pericarditis rather than cirrhosis?
O A) Edema of the lower extremities
B) Esophageal varices
C) Hypoalbuminemia
D) Hyponatremia
, E) Increased jugular venous pressure
F) Splenomegaly
- correct answer E.
Increased jugular venous pressure (JVP) is an expected finding in constrictive pericarditis (CP) and would
not be found in patients with volume overload secondary to cirrhosis. CP describes a pathologic state
whereby the pericardium, which encases the entirety of
the heart and the origins of the great vessels, loses its elasticity. This can occur in patients with viral
infections, connective tissue disease, tuberculosis, or as a result of cardiac surgery or radiation. During
the normal cardiac cycle, increased venous return to the
right atrium (RA) and right ventricle (RV) during inspiration leads to transient expansion of the RV with
slight bowing of the interventricular septum into the left ventricle (LV). This increased RV preload does
not impair LV filling as the pericardial sac expands to
accommodate the increased RV volume. In constrictive pericarditis, the pericardial sac loses its elasticity.
When venous return to the right heart increases, the pericardial sac is unable to expand, which
exacerbates movement of the interventricular septum into the
LV. This impairs diastolic filling of the LV and reduces cardiac output. LV diastolic filling is further
reduced by a reduction in preload from the pulmonary veins. The constricted pericardium does not
respond to normal changes in intrathoracic pressure during
inspiration, but the pulmonary venous system, which lies outside of the pericardium, experiences a
normal drop in pressure during inspiration. This difference creates an abnormal pressure gradient that
reduces LV preload and leads to reduced cardiac output. The
RV and LV develop interventricular dependence whereby increased pressure in each ventricle begins to
affect the other ventricle. CP ultimately results in equalization of pressures in all four chambers, with
clinical evidence of right heart failure exhib
4
----------
Exam Section 1: Item 4 of 50
National Board of Medical Examiners
Comprehensive Basic Science Self-Assessment
4. A male newborn is found to have a defect in anchoring fibrils. Which of the following skin findings is
most likely in this patient?
A) Blisters
B) Easy bruising
