Final answer:
The likely diagnosis is Myasthenia gravis, which is an autoimmune disorder that affects the neuromuscular junction and causes muscle weakness. It is associated with thymoma in some cases.
Step-by-step explanation:
The likely diagnosis for a 31-year-old man with difficulty chewing, intermittent double vision, and mild ptosis of the right eye is Myasthenia gravis. Myasthenia gravis is an autoimmune disorder that affects the neuromuscular junction. It is associated with the blockage of nicotinic acetylcholine receptors, leading to muscle weakness. The initial symptom is painless muscle weakness, often starting in the muscles around the eye. As the disease progresses, it can affect other muscles, including those involved in chewing and swallowing.
Myasthenia gravis is commonly associated with a thymoma, a tumor of the thymus gland. Thymoma can be present in about 10% of patients with myasthenia gravis. It is important to consider the association between myasthenia gravis and thymoma as management and treatment may differ in these cases.