A 61-year-old man comes to the physician for a 2-month history of headache and intermittent left flank pain. He has had a 10-pound weight loss during this time. He has an extensive history of alcoholism, smoking, and illicit drug use. Physical examination shows weakness of the proximal muscles of the upper and lower extremities. The shoulder and hip muscles demonstrate augmentation of strength with repetitive testing. The patient is able to perform upward gaze for 60 seconds without a problem. MRI of the head shows a ring-enhancing lesion in the left frontal lobe. Biopsy shows small blue cells. HIV Elisa is negative. Which of the following is the most likely diagnosis?
Anaplastic ependymoma
Glioblastoma multiforme
Metastatic malignancy
Pilocytic astrocytoma
Primary CNS lymphoma
