Correct Answer :
A. A chest radiograph
The clinical triad of Horners syndromeipsilateral ptosis, miosis, and decreased facial sweating suggests decreased sympathetic innervation due to involvement of the stellate ganglion, a complication of Pancoasts superior sulcus tumors of the lung. Radiographs or MRI of the pulmonary apices and paracervical area is indicated. Horners syndrome may accompany intracranial pathology, such as the lateral medullary syndrome (Wallenbergs syndrome), but is associated with multiple other neurologic symptoms, so MRI of the brain is not indicated at this point. The acetylcholine receptor antibody level is a test for myasthenia gravis, which can also present with ptosis, but not with full-blown Horners syndrome. Diabetes mellitus and thyroid disease do not commonly present with Horners syndrome.