A 52-year-old woman presents with progressive ptosis, diplopia, and fatigability over 3 months. Serum antibodies against acetylcholine receptors are positive. A lateral chest X-ray is performed as part of the diagnostic workup. The retrosternal clear space marked **A** shows increased opacification. Which of the following is the most likely diagnosis?

## Why Thymoma with associated myasthenia gravis is right The patient presents with classic myasthenia gravis (ptosis, diplopia, fatigability) confirmed by positive anti-acetylcholine receptor antibodies. Increased opacification of the retrosternal clear space (anterior mediastinum, marked **A**) on lateral CXR indicates an anterior mediastinal mass. Thymoma is the most common anterior mediastinal mass (30–50% of cases) and is strongly associated with myasthenia gravis. Approximately 30–50% of thymoma patients have concurrent MG, and conversely, 10–15% of MG patients harbor a thymoma. The combination of MG serology and anterior mediastinal opacification makes thymoma the diagnosis of choice. Diagnosis is confirmed by contrast-enhanced CT chest, and treatment is surgical resection, which may improve MG symptoms (Gray's Anatomy 42e, Ch 56). ## Why each distractor is wrong - **Retrosternal goiter with thyroid dysfunction**: While retrosternal goiter is a classic cause of anterior mediastinal opacification, it would not explain positive anti-ACh receptor antibodies or the neurological presentation of myasthenia gravis. Thyroid autoimmunity (Hashimoto's) is sometimes associated with MG, but the direct pathogenic link is the thymoma, not the goiter. - **Anterior mediastinal lymphoma (Hodgkin type)**: Although Hodgkin lymphoma (nodular sclerosis subtype) is a classic anterior mediastinal mass, it would not cause anti-ACh receptor antibodies or the specific neurological syndrome of MG. Lymphoma typically presents with B symptoms, constitutional symptoms, and elevated LDH, not autoimmune neurological disease. - **Mediastinal teratoma with elevated β-hCG**: Teratomas are the most common mediastinal germ cell tumors and do cause anterior mediastinal opacification, but they are not associated with myasthenia gravis or anti-ACh receptor antibodies. Teratomas present with mass effect or, if malignant, with elevated tumor markers (α-FP, β-hCG, LDH), not autoimmune phenomena. **High-Yield:** Anterior mediastinal mass + myasthenia gravis = thymoma until proven otherwise; thymectomy may improve MG symptoms even if thymoma is not present. [cite: Gray's Anatomy 42e, Ch 56]