A patient with fluctuating weakness and a mediastinal mass tests positive for which antibody?

• A. Rapid plasma reagin
• B. Acetylcholine receptor antibody
• C. C-reactive protein
• D. Rheumatoid factor

Answer: (B)

The fluctuating weakness with a mediastinal mass points to myasthenia gravis, an autoimmune disorder of the neuromuscular junction. In MG, antibodies target postsynaptic nicotinic acetylcholine receptors at the motor endplate, reducing receptor availability and impairing acetylcholine signaling. This leads to fatigable weakness that worsens with activity and improves with rest. Thymic abnormalities, especially thymoma, are commonly associated and can drive the autoimmunity, which explains the mediastinal mass. Therefore, the antibody most characteristically detected is the acetylcholine receptor antibody. The other listed markers (RPR, CRP, RF) are not specific to MG or thymic tumor and do not explain the neuromuscular junction–level weakness and mass in this scenario.