A 58-year-old man with progressive exertional dyspnea, orthopnea, and progressive limb weakness with visible fasciculations and upper motor neuron signs undergoes spirometry. The pattern marked B in the diagram shows FVC 45% predicted, FEV1 47% predicted, FEV1/FVC 0.85, with reduced maximal inspiratory pressure (MIP −22 cmH₂O) and maximal expiratory pressure (MEP 38 cmH₂O). Supine FVC drops to 32% predicted. Which of the following neuromuscular diagnoses best explains this restrictive spirometry pattern with disproportionate respiratory muscle weakness?

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