NBME Gross Anatomy and Embryology 2 — Questions and Answers
Question 1: A patient presents with inability to abduct the arm beyond 15 degrees after a humeral surgical neck fracture. Which nerve is most likely injured?
- Radial nerve
- Axillary nerve (Correct answer)
- Musculocutaneous nerve
- Suprascapular nerve
Correct answer: Axillary nerve
The axillary nerve wraps around the surgical neck of the humerus and innervates the deltoid, which is responsible for arm abduction beyond 15 degrees.
Question 2: During embryonic development, failure of the pleuroperitoneal membrane to close results in which congenital defect?
- Esophageal atresia
- Congenital diaphragmatic hernia (Correct answer)
- Omphalocele
- Gastroschisis
Correct answer: Congenital diaphragmatic hernia
Failure of the pleuroperitoneal membranes to fuse closes off the pericardioperitoneal canals, and incomplete closure allows abdominal viscera to herniate into the thorax.
Question 3: A patient with Colles fracture develops inability to oppose the thumb. Which structure is most likely compressed at the wrist?
- Ulnar nerve in Guyon canal
- Median nerve in carpal tunnel (Correct answer)
- Radial nerve at anatomical snuffbox
- Anterior interosseous nerve
Correct answer: Median nerve in carpal tunnel
Colles fracture can cause swelling that compresses the median nerve in the carpal tunnel, impairing thenar muscles responsible for thumb opposition.
Question 4: Which embryonic structure gives rise to the adrenal cortex?
- Neural crest cells
- Intermediate mesoderm (Correct answer)
- Lateral plate mesoderm
- Endoderm of the urogenital sinus
Correct answer: Intermediate mesoderm
The adrenal cortex derives from intermediate mesoderm (specifically the mesothelium lining the posterior abdominal wall), while the medulla derives from neural crest cells.
Question 5: A stab wound to the posterior triangle of the neck results in weakness of shoulder elevation and head rotation. Which nerve is damaged?
- Phrenic nerve
- Accessory nerve (CN XI) (Correct answer)
- Long thoracic nerve
- Dorsal scapular nerve
Correct answer: Accessory nerve (CN XI)
The accessory nerve (CN XI) traverses the posterior triangle and innervates the trapezius (shoulder elevation) and sternocleidomastoid (head rotation).
Question 6: During cardiac embryology, the interventricular septum forms primarily from which structure?
- Endocardial cushions alone
- Muscular septum growing from apex plus membranous portion from cushions (Correct answer)
- Septum primum growing downward
- Neural crest cells migrating into the heart
Correct answer: Muscular septum growing from apex plus membranous portion from cushions
The muscular interventricular septum grows up from the ventricular apex, and the membranous portion is completed by tissue from the endocardial cushions (atrioventricular septal tissue).
Question 7: A patient cannot extend the wrist or fingers after a midshaft humeral fracture. Which nerve is injured?
- Median nerve
- Ulnar nerve
- Radial nerve (Correct answer)
- Musculocutaneous nerve
Correct answer: Radial nerve
The radial nerve runs in the spiral groove of the mid-humerus and innervates all wrist and finger extensors; midshaft fractures commonly injure it.
NBME Gross Anatomy and Embryology 2
A patient presents with inability to abduct the arm beyond 15 degrees after a humeral surgical neck fracture.
Which nerve is most likely injured?