Which of the following statements about the relationship of the thymus and myasthenia gravis is/are true?

A. Thymic abnormalities are present in up to 80% of patients with myasthenia gravis.

B. Thymoma is present in up to 20% of patients with myasthenia gravis.

C. Myasthenia gravis will occur in up to 60% of patients with thymomas.

DISCUSSION: The central role of the thymus gland in the pathogenesis of myasthenia gravis is based on the observation that more than 80% of patients have histologic abnormalities of the thymus and on the beneficial effect of thymectomy on patients' symptoms. Of the patients with documented abnormalities of the thymus the majority have B-cell lymphoid hyperplasia; only 20% have a thymoma. Conversely, up to 60% of patients with known thymoma will have or ultimately develop myasthenia gravis. In these patients, with thymoma and myasthenia gravis, the response to thymectomy is less favorable than in those without thymoma.