Gross anatomy and embryology for USMLE Step 1

A 26-year-old man sustains a mid-shaft humeral fracture in a fall. On examination he cannot extend his wrist or fingers, and there is sensory loss over the dorsal aspect of the first web space. Elbow flexion and forearm supination are preserved. Which nerve is injured?

1. Radial nerve correct Correct. The radial nerve runs in the spiral groove against the humeral shaft, making it the classic casualty of a mid-shaft fracture. It supplies the wrist and finger extensors, giving wrist drop, and its superficial branch supplies the dorsal first web space.
2. Musculocutaneous nerve Supplies the elbow flexors and supinator function via biceps. Elbow flexion and supination are explicitly preserved here.
3. Axillary nerve Injured with surgical neck fractures and shoulder dislocation, giving deltoid weakness and sensory loss over the regimental badge area, not wrist drop.
4. Ulnar nerve Injured at the medial epicondyle or the wrist, producing clawing of the ring and little fingers and loss of interossei function, with medial sensory loss.
5. Median nerve Injury gives loss of thumb opposition and flexion with sensory loss over the lateral palm, not wrist drop. It is more typically injured at the wrist or with supracondylar fractures.

The point: Map the humerus to the nerve: surgical neck to axillary, mid-shaft spiral groove to radial, supracondylar to median, medial epicondyle to ulnar. Radial nerve injury spares triceps if it occurs distal to the branches supplying it, which is why elbow extension can be preserved.

Source: USMLE Content Outline — gross anatomy NBME / FSMB (USMLE programme) · tier 0, exam blueprint / regulator

A 64-year-old man has sudden right-sided weakness of the arm and leg with sparing of the face, loss of vibration and proprioception on the right, and deviation of the tongue to the LEFT on protrusion. Pain and temperature sensation are intact. Where is the lesion?

1. Left lateral medulla Wallenberg syndrome from PICA occlusion gives contralateral body pain and temperature loss, ipsilateral facial pain and temperature loss, Horner syndrome, vertigo and dysphagia, with no limb weakness.
2. Left medial medulla correct Correct. Medial medullary syndrome from anterior spinal artery or vertebral occlusion. Three midline structures: corticospinal tract (contralateral arm and leg weakness), medial lemniscus (contralateral vibration and proprioception loss) and the hypoglossal nucleus (IPSILATERAL tongue deviation, toward the lesion).
3. Left midbrain (Weber syndrome) Gives contralateral hemiparesis WITH an ipsilateral third nerve palsy, a down-and-out eye with a dilated pupil, absent here.
4. Right cervical spinal cord hemisection Brown-Séquard would give ipsilateral weakness and dorsal column loss with CONTRALATERAL pain and temperature loss, and would not affect the tongue.
5. Right internal capsule A pure motor stroke would weaken the LEFT side and would involve the face, with no tongue deviation of this pattern.

The point: Medial brainstem = the three Ms: Motor (corticospinal), Medial lemniscus, Motor cranial nerve nucleus (XII in medulla, VI in pons, III in midbrain). Lateral = the four Ss: Spinothalamic, Sympathetic, Spinal trigeminal, Spinocerebellar.

Source: USMLE Content Outline — neuroanatomy NBME / FSMB (USMLE programme) · tier 0, exam blueprint / regulator

A newborn boy has a defect in which abdominal contents protrude through the abdominal wall to the RIGHT of an intact umbilical cord. There is no covering membrane and the bowel is thickened and matted. What is the embryologic basis?

1. Failure of closure of the lateral body folds, so bowel herniates through a paraumbilical wall defect correct Correct. Gastroschisis. Because there is no covering sac, the exposed bowel is inflamed by amniotic fluid and it is not associated with chromosomal abnormalities.
2. Failure of the pleuroperitoneal folds to fuse Congenital diaphragmatic hernia: bowel in the CHEST with pulmonary hypoplasia, usually left-sided.
3. Failure of the physiologic midgut herniation to return to the abdomen, with a peritoneal covering Omphalocele: the defect is at the umbilicus, the contents are covered by peritoneum and amnion, and it carries a high rate of trisomy 13, 18 and 21 and cardiac anomalies.
4. Persistence of the vitelline duct Gives Meckel diverticulum, or a patent vitelline fistula, not an anterior wall defect of this kind.
5. Incomplete obliteration of the urachus Produces urine drainage from the umbilicus or a urachal cyst, not evisceration.

The point: Gastroschisis: to the right of the cord, NO sac, not syndromic, bowel inflamed. Omphalocele: at the cord, HAS a sac, strongly associated with aneuploidy, so karyotype and echocardiography matter more there.

Source: USMLE Content Outline — embryology NBME / FSMB (USMLE programme) · tier 0, exam blueprint / regulator