Final answer:
The most likely diagnosis for a 50-year-old man with progressive muscle weakness, double vision, and a mass in the anterior mediastinum is myasthenia gravis. This condition is a neuromuscular disorder where the communication between nerves and muscles is impaired, often treatable but not curable.
Step-by-step explanation:
A 50-year-old man presenting with progressive weakness of the upper and lower extremities and double vision, with a radiograph revealing a mass in the anterior mediastinum, is most likely suffering from myasthenia gravis (MG). This neuromuscular disorder is characterized by the weakness and rapid fatigue of any of the muscles under voluntary control. MG is caused by a breakdown in the normal communication between nerves and muscles. There is no cure for myasthenia gravis, but treatment can help relieve signs and symptoms, such as weakness of arm or leg muscles, double vision, drooping eyelids, and difficulties with speech, chewing, swallowing, and breathing.
Though the historical references and clinical details provided span a range of symptoms and conditions, the mass in the anterior mediastinum and symptoms such as double vision, and progressive limb weakness are classic representations of myasthenia gravis, which can sometimes be associated with thymomas or thymic hyperplasia typically found within the anterior mediastinum. A confirmatory diagnostic approach could involve clinical examination, acetylcholine receptor antibody testing, and, when indicated, imaging studies such as a CT scan or MRI to evaluate the thymus gland, in addition to electrophysiological studies or a trial of medications that improve neuromuscular transmission (e.g., anticholinesterase agents).