Three months ago, a 53 year old woman had surgery and chemotherapy for breast cancer. Four weeks ago, she developed a cough occasionally productive of purulent sputum. About 2 weeks ago, she noted a slight but progressive weakness of her left arm and leg. On chest examination, rales were heard over the left upper back when the patient breathed deeply. Neurologic examination confirmed weakness of the left arm and leg. Chest radiography showed a left upper lobe infiltrate. Contrast enhanced computed tomography showed two lesions in the right hemisphere. Gram stain of a purulent sputum specimen showed branching Gram positive rods that were partially acid fast. Which of the following organisms is the cause of this patient's current illness?