A comatose patient has both upper limbs held in flexion at the elbows and wrists with the lower limbs extended. The lesion responsible for this pattern lies above which structure, allowing its descending output to dominate limb tone?
- A Lateral vestibular nucleus
- B Red nucleus ✓
- C Reticular formation of the pons
- D Vestibular nuclei of the medulla
Correct answer: B. Red nucleus
Explanation
Decorticate (flexor) posturing occurs when the lesion is above the red nucleus, preserving the rubrospinal tract, which facilitates flexor tone in the upper limbs. B lesion below the red nucleus removes rubrospinal influence, leaving unopposed vestibulospinal and reticulospinal drive to extensors, producing decerebrate (extensor) posturing. The lateral vestibular nucleus and pontomedullary reticular formation promote extension, so their preservation alone cannot explain upper limb flexion.
Reference: Ganong's Review of Medical Physiology, 26th ed.
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