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 a groove (the spiral groove) that wraps around the shaft of the humerus bone, which is why it is the nerve most often injured in a mid-shaft humerus fracture. It supplies the muscles that extend the wrist and fingers, so damage causes wrist drop, and its surface branch supplies feeling over the back of the first web space, between the thumb and index finger.
2. Musculocutaneous nerve Wrong. The musculocutaneous nerve supplies the muscles that bend the elbow and, through the biceps, helps turn the forearm palm-up (supination). The question states that elbow flexion and supination are both still working, so this nerve is not the one injured.
3. Axillary nerve Wrong. The axillary nerve is injured with fractures near the top of the humerus (surgical neck) or shoulder dislocation. This causes weakness of the deltoid muscle and numbness over the outer shoulder (the regimental badge area), not wrist drop.
4. Ulnar nerve Wrong. The ulnar nerve is usually injured at the bony bump on the inner elbow (medial epicondyle) or at the wrist. This causes clawing of the ring and little fingers, loss of the small hand muscles (interossei), and numbness on the inner side of the hand.
5. Median nerve Wrong. Median nerve injury causes loss of thumb movement toward the fingers (opposition) and finger flexion, with numbness over the outer (lateral) palm, not wrist drop. It is more often injured at the wrist or with a different type of fracture near the elbow (supracondylar fracture).

The point: Match the part of the humerus bone to the nerve at risk: the top of the bone (surgical neck) goes with the axillary nerve, the shaft (spiral groove) goes with the radial nerve, the area just above the elbow (supracondylar) goes with the median nerve, and the inner elbow bump (medial epicondyle) goes with the ulnar nerve. A radial nerve injury spares the triceps muscle if it happens below the branches that supply the triceps, which is why elbow extension can still work even with a radial nerve injury.

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 Wrong. A lesion of the left LATERAL medulla causes Wallenberg syndrome, from a blocked PICA artery. It causes loss of pain and temperature sensation on the opposite side of the body, loss of pain and temperature sensation on the same side of the face, Horner syndrome, dizziness (vertigo), and trouble swallowing (dysphagia), but no limb weakness.
2. Left medial medulla correct Correct. This is medial medullary syndrome, from a blocked anterior spinal artery or vertebral artery. Three structures sit near the midline here: the corticospinal tract (causing weakness on the OPPOSITE side, arm and leg), the medial lemniscus (causing loss of vibration and joint-position sense on the opposite side), and the hypoglossal nerve nucleus (causing the tongue to deviate toward the SAME side as the lesion when the patient sticks it out).
3. Left midbrain (Weber syndrome) Wrong. A lesion in the left midbrain causes Weber syndrome, with weakness on the opposite side of the body PLUS a third cranial nerve palsy on the same side, causing a 'down and out' eye with a widened pupil, which is not present here.
4. Right cervical spinal cord hemisection Wrong. A right-sided spinal cord hemisection would cause Brown-Séquard syndrome, with weakness and loss of vibration and joint-position sense on the SAME side, and loss of pain and temperature sensation on the OPPOSITE side. It would not affect the tongue at all.
5. Right internal capsule Wrong. A stroke in the right internal capsule causing a pure motor problem would weaken the LEFT side of the body and would involve the face too, without this specific tongue deviation pattern.

The point: The medial brainstem follows the three Ms: Motor pathway (corticospinal tract), Medial lemniscus, and a Motor cranial nerve nucleus (cranial nerve XII in the medulla, VI in the pons, III in the midbrain). The lateral brainstem follows the four Ss: Spinothalamic tract, Sympathetic fibers, Spinal trigeminal nucleus, and Spinocerebellar tract.

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. This is gastroschisis. Because there is no covering sac protecting the bowel, the exposed bowel gets irritated by the amniotic fluid around it, and gastroschisis is not linked to chromosome abnormalities.
2. Failure of the pleuroperitoneal folds to fuse Wrong. Failure of the pleuroperitoneal folds to close causes a congenital diaphragmatic hernia, with bowel pushed up into the CHEST and underdeveloped lungs (pulmonary hypoplasia), usually on the left side.
3. Failure of the physiologic midgut herniation to return to the abdomen, with a peritoneal covering Wrong. Failure of the normal bulging-out of the midgut (physiologic midgut herniation) to return to the belly, with a covering layer, describes an omphalocele. There, the defect is right at the umbilicus, the contents are covered by a membrane (peritoneum and amnion), and it is strongly linked to trisomy 13, 18, and 21, plus heart defects.
4. Persistence of the vitelline duct Wrong. A leftover vitelline duct causes Meckel diverticulum or a vitelline fistula, not this kind of abdominal wall defect.
5. Incomplete obliteration of the urachus Wrong. An incompletely closed urachus causes urine to drain from the umbilicus, or a urachal cyst, not bowel spilling out of the body (evisceration).

The point: Gastroschisis: located to the right of the umbilical cord, NO covering sac, not linked to genetic syndromes, and the exposed bowel is inflamed. Omphalocele: located right at the cord, HAS a covering sac, and is strongly linked to extra or missing chromosomes, which is why checking the baby's chromosomes (karyotype) and heart with echocardiography matters much more there.

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