NBME 26 ACTUAL EXAM 208
QUESTIONS AND ANSWERS (LATEST
2026/2027 UPDATE) COMPLETE EXAM
(usmle step 1) MEDICAL EXAMINATION
This NBME 26 is a Comprehensive Basic Science Self-Assessment designed for USMLE Step
1 preparation, consisting of 200+ multiple-choice questions (4 blocks of 50) covering
foundational medical sciences. It focuses on high-yield topics like [Lesch-Nyhan syndrome],
[cardiac physiology], [pathology (compartment syndrome)], and [microbiology/infectious
disease], offering detailed rationales for correct answers and distractor analysis to build
diagnostic reasoning.
Key Aspects of NBME 26 Questions & Rationales:
Structure: Questions typically feature clinical vignettes requiring diagnosis, mechanism of
action, or next best step, followed by one correct answer and several distractors.
Key Topics Covered:
Metabolism: Purine/pyrimidine salvage pathways (e.g., HGPRT dysfunction in Lesch-
Nyhan).
Pathology: Compartment syndrome following fractures, leading to rhabdomyolysis and
ATN.
Cardiovascular: Pathophysiology of constrictive pericarditis vs. cirrhosis, and cardiac
cycle/ventricular pressure graphs.
Microbiology/Immune: Viral infections (Hepatitis A) and immune mechanisms.
Histology: Connective tissue disorders (anchoring fibril defects).
Rationales & Study Focus:
The rationales emphasize why a correct answer is correct (e.g., distinguishing RA vs. OA
symptoms) rather than just stating it.
Distractors are explained to show why they are less likely, improving understanding of
similar conditions.
High-yield topics often reappear in different forms in subsequent NBME exams.
These questions and their explanations are synthesized by Experts
1
, Quiz_________________?
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 -
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
2
, 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
Quiz_________________?
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 -
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
3
, 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
Quiz_________________?
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
4
QUESTIONS AND ANSWERS (LATEST
2026/2027 UPDATE) COMPLETE EXAM
(usmle step 1) MEDICAL EXAMINATION
This NBME 26 is a Comprehensive Basic Science Self-Assessment designed for USMLE Step
1 preparation, consisting of 200+ multiple-choice questions (4 blocks of 50) covering
foundational medical sciences. It focuses on high-yield topics like [Lesch-Nyhan syndrome],
[cardiac physiology], [pathology (compartment syndrome)], and [microbiology/infectious
disease], offering detailed rationales for correct answers and distractor analysis to build
diagnostic reasoning.
Key Aspects of NBME 26 Questions & Rationales:
Structure: Questions typically feature clinical vignettes requiring diagnosis, mechanism of
action, or next best step, followed by one correct answer and several distractors.
Key Topics Covered:
Metabolism: Purine/pyrimidine salvage pathways (e.g., HGPRT dysfunction in Lesch-
Nyhan).
Pathology: Compartment syndrome following fractures, leading to rhabdomyolysis and
ATN.
Cardiovascular: Pathophysiology of constrictive pericarditis vs. cirrhosis, and cardiac
cycle/ventricular pressure graphs.
Microbiology/Immune: Viral infections (Hepatitis A) and immune mechanisms.
Histology: Connective tissue disorders (anchoring fibril defects).
Rationales & Study Focus:
The rationales emphasize why a correct answer is correct (e.g., distinguishing RA vs. OA
symptoms) rather than just stating it.
Distractors are explained to show why they are less likely, improving understanding of
similar conditions.
High-yield topics often reappear in different forms in subsequent NBME exams.
These questions and their explanations are synthesized by Experts
1
, Quiz_________________?
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 -
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
2
, 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
Quiz_________________?
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 -
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
3
, 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
Quiz_________________?
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
4
