**Core Concept**
Myasthenia Gravis (MG) is an autoimmune disease characterized by the production of antibodies against the acetylcholine receptor (AChR) at the neuromuscular junction, leading to impaired transmission of nerve impulses to muscles. This results in fluctuating muscle weakness and fatigue, which worsens with repeated use.

**Why the Correct Answer is Right**
The symptoms described in the patient, such as diplopia, ptosis, and weakness of muscles on repeated use, are classic manifestations of MG. The presence of antibodies against the AChR at the neuromuscular junction disrupts the normal process of acetylcholine binding to the receptor, leading to a decrease in muscle contraction and subsequent symptoms. The use of repeated nerve impulses to stimulate muscles (fatigability) exacerbates the condition.

**Why Each Wrong Option is Incorrect**
**Option A:** Lambert-Eaton Myasthenic Syndrome (LEMS) is a rare autoimmune disorder that involves antibodies against the voltage-gated calcium channels at the neuromuscular junction, causing muscle weakness that improves with repeated use, which is opposite to the symptoms described.

**Option B:** Guillain-Barre Syndrome is an autoimmune disorder that involves demyelination of peripheral nerves, leading to ascending muscle weakness and paralysis, which is not directly related to the symptoms described.

**Option C:** Dermatomyositis is an inflammatory muscle disease characterized by skin and muscle involvement, which may present with weakness and fatigue, but the symptoms described are more characteristic of MG.

**Option D:** This option is empty, implying that the question had a blank space for the options.

**Clinical Pearl / High-Yield Fact**
Myasthenia Gravis is a classic example of an autoimmune disease that can mimic other neurological conditions. A key clinical clue is the presence of diplopia, which is often the first symptom to appear, followed by weakness and fatigue of other muscles.

**Correct Answer: C. Dermatomyositis**