## Why option 1 is right Chiari I malformation is defined by cerebellar tonsillar displacement >5 mm below the foramen magnum (as shown by structure **A** in the diagram). The associated syringomyelia—a CSF-filled cavity within the spinal cord—characteristically disrupts crossing spinothalamic fibers in the anterior white commissure, producing the pathognomonic "cape-like" bilateral loss of pain and temperature sensation in the upper extremities with preserved proprioception. This dissociated sensory loss is the hallmark of syrinx-related myelopathy and directly results from interruption of the crossing pain/temperature fibers while dorsal columns remain intact. Harrison 21e Ch 446 emphasizes this clinical-radiological correlation. ## Why each distractor is wrong - **Option 2**: Dorsal column involvement causes loss of vibration sense and proprioception, not pain and temperature loss. The patient's preserved proprioception explicitly excludes dorsal column pathology. - **Option 3**: Corticospinal tract damage produces upper motor neuron weakness and hyperreflexia, not dissociated sensory loss. The clinical presentation is purely sensory, not motor. - **Option 4**: Posterior root entry zone lesions cause dermatomal radicular pain, not the bilateral cape-like distribution seen here. Syringomyelia produces centrally-located sensory loss, not root-level pathology. **High-Yield:** Chiari I + syringomyelia = cape-like bilateral upper extremity pain/temperature loss with preserved proprioception (spinothalamic disruption in anterior commissure). [cite: Harrison 21e Ch 446]
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