Myasthenia Gravis (MG): This is the most likely diagnosis given the presence of acetylcholine receptor antibodies, which are found in approximately 85-90% of patients with MG. The disease is characterized by fluctuating muscle weakness that worsens with activity and improves with rest. The presence of anti-striated muscle antibodies can also be seen in MG, particularly in patients with thymoma.

Thymoma-associated Myasthenia Gravis: Thymoma is a tumor of the thymus gland and is associated with MG in about 10-15% of cases. Patients with thymoma are more likely to have anti-striated muscle antibodies, making this a likely consideration.
Lambert-Eaton Myasthenic Syndrome (LEMS): Although less common, LEMS can present with similar symptoms to MG and may have positive acetylcholine receptor antibodies in some cases. However, the primary antibody associated with LEMS is the voltage-gated calcium channel antibody.

Paraneoplastic Syndromes: It's crucial not to miss paraneoplastic syndromes, which are conditions caused by the immune response to a cancer, but not due to direct local effects of the tumor cells. These can include LEMS or other neuromuscular disorders associated with various cancers. The presence of anti-striated muscle antibodies might suggest an underlying malignancy.
Thymic Carcinoma: Although rare, thymic carcinoma can be associated with MG and the presence of anti-striated muscle antibodies. Given its aggressive nature and poor prognosis, it's essential to consider and rule out this diagnosis.

Autoimmune Myopathies: Certain autoimmune myopathies, such as polymyositis or dermatomyositis, can rarely present with positive acetylcholine receptor antibodies or anti-striated muscle antibodies, although this is not typical.
Neuromuscular Junction Disorders Other Than MG or LEMS: There are other, rarer disorders of the neuromuscular junction that might present with similar antibody profiles, such as congenital myasthenic syndromes, though these are typically identified earlier in life.