NBME 26 Exam Questions with Verified
Correct Answers
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
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 dystonia. Mutations in HGPRT cause this
disorder by impairing salvage of the purines guanine and hypoxanthine, which leads to
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
synthes
of uric acid.
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
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 hematuria, red blood cell urine casts, and hypertension. Nephrotic
syndrome typically presents with excessive proteinuria (greater than 3 g/day),
hyperlipidemia, hypoalbuminemia, and edema.
Hemolytic-uremic syndrome (Choice B) is classically associated with infection from
Escherichia coli 0157:H7, a Gram-negative rod. It often presents in children with bloody
diarrhea, microangiopathic hemolytic anemia, thrombocytopenia, and renal failure. It is not
associated with trauma or compartment syndrome.
Interstitial nephritis (Choice C) is usually caused by a hypersensitivity reaction to offending
drugs (eg, nonsteroidal anti-inflammatory medications, diuretics, and sulfonamides) and is
characterized by a rash, acute kidney injury, and eosinophiluria.
Nephrotic syndrome (Choice D) is characterized by proteinuria (greater than 3 g/day), edema,
hypoalbuminemia, and hyperlipidemia. Complications include hypercoagulability due to
urinary loss of antithrombin-3 and infection due to urinary loss of gamma
Correct Answers
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
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 dystonia. Mutations in HGPRT cause this
disorder by impairing salvage of the purines guanine and hypoxanthine, which leads to
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
synthes
of uric acid.
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
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 hematuria, red blood cell urine casts, and hypertension. Nephrotic
syndrome typically presents with excessive proteinuria (greater than 3 g/day),
hyperlipidemia, hypoalbuminemia, and edema.
Hemolytic-uremic syndrome (Choice B) is classically associated with infection from
Escherichia coli 0157:H7, a Gram-negative rod. It often presents in children with bloody
diarrhea, microangiopathic hemolytic anemia, thrombocytopenia, and renal failure. It is not
associated with trauma or compartment syndrome.
Interstitial nephritis (Choice C) is usually caused by a hypersensitivity reaction to offending
drugs (eg, nonsteroidal anti-inflammatory medications, diuretics, and sulfonamides) and is
characterized by a rash, acute kidney injury, and eosinophiluria.
Nephrotic syndrome (Choice D) is characterized by proteinuria (greater than 3 g/day), edema,
hypoalbuminemia, and hyperlipidemia. Complications include hypercoagulability due to
urinary loss of antithrombin-3 and infection due to urinary loss of gamma
