Clinical features of myasthenia gravis:

Muscle weakness and fatiguability increase with repeated use or late in the day and may improve following rest or sleep. Limb weakness is proximal and asymmetrical

Cranial muscles particularly the lids and extraocular muscles are involved early in the course - diplopia, ptosis
Facial weakness - snarling expression on smiling
Weakness in chewing
Speech with nasal timbre due to the weakness of the palate and tongue
Difficulty in swallowing
Deep tendon reflexes are intact
Myasthenic crisis - acute exacerbation of muscle weakness especially intercostal and diaphragmatic muscles resulting in ventilatory failure. It is most commonly due to infection and anticholinesterase medication overuse.

Myasthenia gravis is associated with thymoma and other autoimmune diseases such as Hashimoto's thyroiditis, Graves' disease, neuromyelitis optics, and systemic lupus erythematosus.

Other options:

Option A: Duchenne muscular dystrophy (DMD) is X-linked muscular dystrophy due to mutations in the dystrophin gene. It most commonly affects boys in their childhood. Patients initially present with proximal muscle weakness predominantly in the lower limbs. Calf muscle hypertrophy is a cardinal feature. Other features include mental retardation, cardiac failure, and myoglobinuria. Elevated serum creatinine kinase levels and genetic testing are diagnostic of DMD.
Option B: Lambert-Eaton myasthenic syndrome (LEMS) is an autoimmune disorder affecting the presynaptic membrane at the neuromuscular junction. Patients present with proximal muscle weakness of the lower limbs, ptosis, and diplopia. However, symptoms might improve after brief exercise. Deep tendon reflexes are absent or reduced. They also experience autonomic dysfunction and present with dry mouth and impotence. The repetitive nerve stimulation test shows an incremental response at higher rates. Treatment involves plasmapheresis, immunotherapy, and drugs such as 3,4-diamino pyridine (3,4-DAP) and pyridostigmine.
Option D: Systemic lupus erythematosus (SLE) is an autoimmune disease that most commonly presents with arthralgia with early morning stiffness. It predominantly occurs in middle-aged women. Other important features of SLE include malar rash, discoid rash, photosensitivity, oral ulcers, pleuritis, pericarditis, and lupus nephritis. Anti-nuclear antibodies, Anti-dsDNA antibodies, and anti-Smith antibodies are positive in SLE.

Reference: Harrison's Principles of Internal Medicine, 21st Edition (Page no 3510-5516)

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