**Core Concept**
Muscle weakness and fatigue, particularly proximal muscles, combined with ptosis, suggest a disorder of muscle function. This presentation is characteristic of a neuromuscular junction disorder, specifically one that affects acetylcholine receptors.

**Why the Correct Answer is Right**
The patient's symptoms are classic for myasthenia gravis (MG), an autoimmune disease that targets the acetylcholine receptor (AChR) at the neuromuscular junction. In MG, autoantibodies against the AChR cause a reduction in the number of functional receptors, leading to impaired neurotransmission and muscle weakness. The symptoms worsen with repeated use of muscles, explaining the easy fatiguability. The presence of mild bilateral ptosis further supports this diagnosis.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not provided, so we cannot comment on it.
* **Option B:** Could be a distractor for other neuromuscular disorders, such as muscular dystrophy or peripheral neuropathy, but does not specifically address the symptoms described.
* **Option C:** This option might be a distractor for other autoimmune diseases, such as rheumatoid arthritis or lupus, but does not directly relate to the neuromuscular junction symptoms.
* **Option D:** This option might be a distractor for other conditions, such as thyroid disorders or metabolic myopathies, but does not specifically address the symptoms described.

**Clinical Pearl / High-Yield Fact**
The "fatigable weakness" in myasthenia gravis is due to the decremental response to repetitive nerve stimulation, which can be demonstrated on electromyography (EMG). This is a key diagnostic clue that distinguishes MG from other neuromuscular disorders.

**Correct Answer:** C. Edrophonium chloride (Tensilon) test.