**Core Concept**
The patient's symptoms, including muscle weakness and drooping eyelids that worsen with repetitive movements, suggest a disorder of the neuromuscular junction. The anterior mediastinal mass on chest X-ray points towards a thymoma, a tumor originating from the epithelial cells of the thymus.

**Why the Correct Answer is Right**
Thymomas are associated with various autoimmune disorders, including myasthenia gravis (MG), an autoimmune disease of the neuromuscular junction. MG is characterized by the presence of antibodies against acetylcholine receptors (AChR), leading to impaired neuromuscular transmission. The symptoms of MG, such as muscle weakness and drooping eyelids, worsen with repetitive movements due to the depletion of AChR. A short rest allows for the replenishment of AChR, temporarily improving symptoms.

**Why Each Wrong Option is Incorrect**
**Option A:** Lymphoma - While lymphoma can also present as an anterior mediastinal mass, it is less commonly associated with MG and the patient's symptoms.

**Option B:** Teratoma - Teratomas are a type of germ cell tumor that can occur in the anterior mediastinum but are not typically associated with MG.

**Option C:** Thymic cyst - Thymic cysts are benign fluid-filled structures that can occur in the thymus but are not associated with MG or the patient's symptoms.

**Clinical Pearl / High-Yield Fact**
Thymomas are often associated with myasthenia gravis, an autoimmune disease of the neuromuscular junction. Patients with thymomas should undergo thorough evaluation for MG, including serological testing for AChR antibodies.

**Correct Answer:** C. Thymoma