The Sound Must Seem an Echo to the Sense

A 51 year old man with hypothyroidism presents with a confusing constellation of symptoms. He's seen multiple physicians without an answer. Five years ago, he began having tingling, paresthesias, and the feeling of coldness in his feet. Over time, he developed weakness in his lower extremities. All these symptoms started distally and had gradual, proximal, symmetric spread. He has no history of diabetes or alcoholism; his B12 and methylmalonic acid are normal. His primary care physician was baffled. A few years later, he could not even climb stairs, arise from a chair, or grip things strongly in either hand. On exam, muscle weakness is more marked than sensory loss, though he has both; his touch, pressure, vibratory, and joint discrimination are more affected than temperature and nociception. There are no cranial nerve abnormalities, though you do note papilledema. A lumbar puncture shows an elevated CSF protein > 100mg/dL with a normal cell count.

Review of systems is remarkable for erectile dysfunction and unintentional weight loss. The rest of the exam is surprising for hyperpigmented skin, telangiectasias, mild hepatosplenomegaly, and lymphadenopathy. Labs show low testosterone and a serum monoclonal protein. An X-ray is shown below.

(Just focus on the arrow; I know this X-ray is a female, not a male)

Challenge: What's your diagnosis?

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