According to the text, Myasthenia gravis is explicitly defined as an 'Auto immune disorder affecting neuromuscular junction leading to fatigue, weakening and paralysis of skeletal muscle'. While Rheumatoid arthritis is an autoimmune disease, the option here is simply 'Arthritis', which is generally defined as inflammation of joints. Osteoporosis is an age-related disorder, and Gout is due to the accumulation of uric acid crystals.