Which disorder is associated with muscle weakness and rapid fatigue?

Myasthenia gravis is indeed the disorder associated with muscle weakness and rapid fatigue. This condition is characterized by an autoimmune response where the body's immune system mistakenly attacks the acetylcholine receptors at the neuromuscular junction. As a result, the communication between nerves and muscles is impaired, leading to weakness in various muscle groups. The fatigue experienced is particularly notable because it tends to worsen with activity and improve with rest. This pattern is distinctive in myasthenia gravis.

In contrast, while multiple sclerosis involves muscle weakness, it is more related to the demyelination of nerves affecting multiple areas of the central nervous system and doesn't usually manifest with the rapid fatigue typical of myasthenia gravis. Guillain-Barré syndrome presents as a rapid onset of muscle weakness, often following an infection, but the nature of fatigue in this condition differs significantly from that of myasthenia gravis. Muscular dystrophy involves progressive muscle degeneration and weakness, but it doesn't usually present with rapid fatigue in the same way; rather, it is chronic and progressive muscle deterioration. Thus, myasthenia gravis is the most appropriate choice for the specific symptoms of muscle weakness and rapid fatigue.