1
NBME CBSE 2026/2027 REAL EXAM
(USMLE Step 1) Complete Comprehensive
Basic Science Examination | Actual Questions
& Verified Answers | USMLE Step 1
Preparation | Pass Guarantee
Section 1: Anatomy & Embryology
Q1: A 58-year-old male with a 40-pack-year smoking history presents with coughing up blood
and a 6-month history of chest pain. CT scan reveals a 4-cm mass in the right lung apex invading
adjacent structures. Physical examination shows ptosis, miosis, and anhidrosis on the right side
of the face. Which anatomic structure is most likely being invaded by this tumor?
A. Phrenic nerve
B. Sympathetic trunk
C. Vagus nerve
D. Recurrent laryngeal nerve
Answer: B
Verified Rationale: This is correct because apical lung tumors (Pancoast tumors) classically
invade the sympathetic trunk causing Horner's syndrome (ptosis, miosis, anhidrosis), and the
anatomic location explains the characteristic clinical presentation.
Q2: During a thyroidectomy, the surgeon must carefully preserve a structure that passes deep to
the inferior border of the thyroid gland and lies superficial to the prevertebral fascia. Injury to
this structure would result in difficulty with neck flexion against resistance. Which structure is
being described?
A. Ansa cervicalis
B. Spinal accessory nerve
C. Superior laryngeal nerve
D. Transverse cervical nerve
Answer: B
,2
Verified Rationale: This is correct because the spinal accessory nerve (CN XI) passes deep to the
inferior thyroid pole and innervates the sternocleidomastoid and trapezius muscles, with injury
causing weakness in neck flexion against resistance.
Q3: A 32-year-old woman presents with difficulty swallowing both solids and liquids. Barium
swallow reveals a dilated esophagus with a "bird's beak" tapering at the lower esophageal
sphincter. Which muscle layer is primarily affected in this condition?
A. Circular muscle layer of the upper third of esophagus
B. Longitudinal muscle layer of the middle third of esophagus
C. Myenteric plexus of the lower two-thirds of esophagus
D. Muscularis mucosa throughout the esophagus
Answer: C
Verified Rationale: This is correct because achalasia results from loss of inhibitory neurons in the
myenteric (Auerbach's) plexus of the lower esophageal sphincter, preventing relaxation and
causing the characteristic "bird's beak" appearance.
Q4: A 45-year-old male suffers a stab wound to the left side of the chest at the fifth intercostal
space, mid-axillary line. He is hypotensive with distended neck veins. Emergency thoracentesis
is performed to evacuate fluid. Which anatomic space is being accessed by needle insertion
through this intercostal space?
A. Cardiac notch
B. Costodiaphragmatic recess
C. Hilar reflection
D. Pulmonary ligament
Answer: B
Verified Rationale: This is correct because the costodiaphragmatic recess (costophrenic angle) is
the lowest part of the pleural cavity where fluid accumulates in a supine patient, and it is
accessed at the mid-axillary line at the 5th-6th intercostal space.
Q5: A 28-year-old female presents with amenorrhea and galactorrhea. MRI reveals a pituitary
adenoma compressing the optic chiasm. Which visual field defect would be expected?
A. Left homonymous hemianopia
B. Right homonymous hemianopia
C. Bitemporal hemianopia
,3
D. Left upper quadrantanopia
Answer: C
Verified Rationale: This is correct because compression of the central portion of the optic chiasm
affects decussating fibers from the nasal retina (temporal visual fields), producing bitemporal
hemianopia, a classic finding in pituitary macroadenomas.
Q6: A neonate is born with a midline neck mass that moves upward with tongue protrusion and
swallowing. Histologic examination shows thyroid follicles within the mass. Failure of which
embryologic structure to properly involute explains this finding?
A. First pharyngeal pouch
B. Second pharyngeal arch
C. Thyroglossal duct
D. Primitive foregut
Answer: C
Verified Rationale: This is correct because the thyroglossal duct normally involutes after thyroid
descent from the foramen cecum, and its persistence results in a thyroglossal duct cyst, which
contains thyroid tissue and moves with tongue protrusion.
Q7: A 67-year-old male presents with sudden onset of right-sided lower back pain radiating to
the groin. CT scan shows an abdominal aortic aneurysm that appears to be compressing a
structure against the psoas major muscle. Which structure is most likely compressed?
A. Right ureter
B. Inferior vena cava
C. Duodenum
D. Pancreas
Answer: A
Verified Rationale: This is correct because the right ureter passes posterior to the lower
abdominal aorta and can be compressed by an expanding aortic aneurysm against the psoas
major muscle, causing flank pain radiating to the groin.
Q8: During development, a 28-week fetus is found to have a congenital diaphragmatic hernia on
prenatal ultrasound. Which embryologic structure failed to close, allowing abdominal contents to
herniate into the thoracic cavity?
A. Pleuroperitoneal canal
, 4
B. Foramen ovale
C. Ductus arteriosus
D. Septum transversum
Answer: A
Verified Rationale: This is correct because the pleuroperitoneal canals (foramina of Bochdalek)
normally close by the ninth week of development, and failure of closure results in congenital
diaphragmatic hernia with abdominal organ herniation into the thorax.
Q9: A 23-year-old male presents with weakness of wrist extension and finger extension after a
fracture of the humerus. Which anatomic relationship explains this nerve injury?
A. The nerve passes through the medial epicondyle
B. The nerve passes through the lateral epicondyle
C. The nerve passes through the spiral groove
D. The nerve passes through the carpal tunnel
Answer: C
Verified Rationale: This is correct because the radial nerve passes through the spiral groove of
the humerus, and midshaft humeral fractures commonly injure this nerve, resulting in wrist drop
and finger extension weakness.
Q10: A 55-year-old female undergoes coronary artery bypass grafting using the great saphenous
vein. Postoperatively, she develops loss of sensation over the medial malleolus and medial foot.
Which nerve was most likely injured during vein harvesting?
A. Deep peroneal nerve
B. Superficial peroneal nerve
C. Saphenous nerve
D. Sural nerve
Answer: C
Verified Rationale: This is correct because the saphenous nerve, a branch of the femoral nerve,
accompanies the great saphenous vein and provides cutaneous innervation to the medial leg and
foot, making it vulnerable during vein harvest.
Q11: A 19-year-old college student experiences sudden onset of severe right testicular pain while
sleeping. Physical examination reveals a high-riding testicle with absent cremasteric reflex.
Which anatomic structure is abnormally twisted in this condition?
NBME CBSE 2026/2027 REAL EXAM
(USMLE Step 1) Complete Comprehensive
Basic Science Examination | Actual Questions
& Verified Answers | USMLE Step 1
Preparation | Pass Guarantee
Section 1: Anatomy & Embryology
Q1: A 58-year-old male with a 40-pack-year smoking history presents with coughing up blood
and a 6-month history of chest pain. CT scan reveals a 4-cm mass in the right lung apex invading
adjacent structures. Physical examination shows ptosis, miosis, and anhidrosis on the right side
of the face. Which anatomic structure is most likely being invaded by this tumor?
A. Phrenic nerve
B. Sympathetic trunk
C. Vagus nerve
D. Recurrent laryngeal nerve
Answer: B
Verified Rationale: This is correct because apical lung tumors (Pancoast tumors) classically
invade the sympathetic trunk causing Horner's syndrome (ptosis, miosis, anhidrosis), and the
anatomic location explains the characteristic clinical presentation.
Q2: During a thyroidectomy, the surgeon must carefully preserve a structure that passes deep to
the inferior border of the thyroid gland and lies superficial to the prevertebral fascia. Injury to
this structure would result in difficulty with neck flexion against resistance. Which structure is
being described?
A. Ansa cervicalis
B. Spinal accessory nerve
C. Superior laryngeal nerve
D. Transverse cervical nerve
Answer: B
,2
Verified Rationale: This is correct because the spinal accessory nerve (CN XI) passes deep to the
inferior thyroid pole and innervates the sternocleidomastoid and trapezius muscles, with injury
causing weakness in neck flexion against resistance.
Q3: A 32-year-old woman presents with difficulty swallowing both solids and liquids. Barium
swallow reveals a dilated esophagus with a "bird's beak" tapering at the lower esophageal
sphincter. Which muscle layer is primarily affected in this condition?
A. Circular muscle layer of the upper third of esophagus
B. Longitudinal muscle layer of the middle third of esophagus
C. Myenteric plexus of the lower two-thirds of esophagus
D. Muscularis mucosa throughout the esophagus
Answer: C
Verified Rationale: This is correct because achalasia results from loss of inhibitory neurons in the
myenteric (Auerbach's) plexus of the lower esophageal sphincter, preventing relaxation and
causing the characteristic "bird's beak" appearance.
Q4: A 45-year-old male suffers a stab wound to the left side of the chest at the fifth intercostal
space, mid-axillary line. He is hypotensive with distended neck veins. Emergency thoracentesis
is performed to evacuate fluid. Which anatomic space is being accessed by needle insertion
through this intercostal space?
A. Cardiac notch
B. Costodiaphragmatic recess
C. Hilar reflection
D. Pulmonary ligament
Answer: B
Verified Rationale: This is correct because the costodiaphragmatic recess (costophrenic angle) is
the lowest part of the pleural cavity where fluid accumulates in a supine patient, and it is
accessed at the mid-axillary line at the 5th-6th intercostal space.
Q5: A 28-year-old female presents with amenorrhea and galactorrhea. MRI reveals a pituitary
adenoma compressing the optic chiasm. Which visual field defect would be expected?
A. Left homonymous hemianopia
B. Right homonymous hemianopia
C. Bitemporal hemianopia
,3
D. Left upper quadrantanopia
Answer: C
Verified Rationale: This is correct because compression of the central portion of the optic chiasm
affects decussating fibers from the nasal retina (temporal visual fields), producing bitemporal
hemianopia, a classic finding in pituitary macroadenomas.
Q6: A neonate is born with a midline neck mass that moves upward with tongue protrusion and
swallowing. Histologic examination shows thyroid follicles within the mass. Failure of which
embryologic structure to properly involute explains this finding?
A. First pharyngeal pouch
B. Second pharyngeal arch
C. Thyroglossal duct
D. Primitive foregut
Answer: C
Verified Rationale: This is correct because the thyroglossal duct normally involutes after thyroid
descent from the foramen cecum, and its persistence results in a thyroglossal duct cyst, which
contains thyroid tissue and moves with tongue protrusion.
Q7: A 67-year-old male presents with sudden onset of right-sided lower back pain radiating to
the groin. CT scan shows an abdominal aortic aneurysm that appears to be compressing a
structure against the psoas major muscle. Which structure is most likely compressed?
A. Right ureter
B. Inferior vena cava
C. Duodenum
D. Pancreas
Answer: A
Verified Rationale: This is correct because the right ureter passes posterior to the lower
abdominal aorta and can be compressed by an expanding aortic aneurysm against the psoas
major muscle, causing flank pain radiating to the groin.
Q8: During development, a 28-week fetus is found to have a congenital diaphragmatic hernia on
prenatal ultrasound. Which embryologic structure failed to close, allowing abdominal contents to
herniate into the thoracic cavity?
A. Pleuroperitoneal canal
, 4
B. Foramen ovale
C. Ductus arteriosus
D. Septum transversum
Answer: A
Verified Rationale: This is correct because the pleuroperitoneal canals (foramina of Bochdalek)
normally close by the ninth week of development, and failure of closure results in congenital
diaphragmatic hernia with abdominal organ herniation into the thorax.
Q9: A 23-year-old male presents with weakness of wrist extension and finger extension after a
fracture of the humerus. Which anatomic relationship explains this nerve injury?
A. The nerve passes through the medial epicondyle
B. The nerve passes through the lateral epicondyle
C. The nerve passes through the spiral groove
D. The nerve passes through the carpal tunnel
Answer: C
Verified Rationale: This is correct because the radial nerve passes through the spiral groove of
the humerus, and midshaft humeral fractures commonly injure this nerve, resulting in wrist drop
and finger extension weakness.
Q10: A 55-year-old female undergoes coronary artery bypass grafting using the great saphenous
vein. Postoperatively, she develops loss of sensation over the medial malleolus and medial foot.
Which nerve was most likely injured during vein harvesting?
A. Deep peroneal nerve
B. Superficial peroneal nerve
C. Saphenous nerve
D. Sural nerve
Answer: C
Verified Rationale: This is correct because the saphenous nerve, a branch of the femoral nerve,
accompanies the great saphenous vein and provides cutaneous innervation to the medial leg and
foot, making it vulnerable during vein harvest.
Q11: A 19-year-old college student experiences sudden onset of severe right testicular pain while
sleeping. Physical examination reveals a high-riding testicle with absent cremasteric reflex.
Which anatomic structure is abnormally twisted in this condition?
