History A 29-year-old woman presented with a 6-month history of progressive general fatigue, fluctuating limb weakness, and difficulty climbing stairs. She initially experienced occasional episodes of transient diplopia that developed while reading in the evening. She subsequently started to experience dry eyes and mouth, difficulty chewing, and mild dysphagia that worsened throughout the day. Her medical history included hypothyroidism from Hashimoto thyroiditis and pneumonia with left pleural effusion. She had no smoking history, and her body mass index was normal (23.8 kg/m2). No medication use was reported at admission. Physical examination revealed mild bilateral ptosis, reduced muscle tone and strength that worsened in proximal leg muscles, and decreased deep tendon reflexes. An edrophonium test revealed improvement in muscle strength and eyelid ptosis. Repetitive nerve stimulation revealed low amplitude of compound muscle action potential at rest (0.21 mV), with a marked increase (700%; normal increase, <60%) at high-rate stimulation (50 Hz). Laboratory work-up was unremarkable except for detection of acetylcholine receptor antibodies in the serum (21.30 nmol/L) and P/Q-type voltage-gated calcium channel antibodies (220 pmol/L). Recent MRI of the brain and spine at an outside hospital showed no abnormal findings. At admission, the patient underwent CT of the chest, abdomen, and pelvis ( Fig 1 ) followed by thoracic MRI ( Figs 2 , 3 ) to further evaluate CT findings. [Figure: see text][Figure: see text][Figure: see text][Figure: see text][Figure: see text][Figure: see text][Figure: see text][Figure: see text].