An 18 year-old female presents after completing her freshman year at college. Two months ago, she developed "bronchitis" which has persisted. She has become increasingly tired and has frequent night sweats. A chest x-ray today reveals a left mediastinal mass. What is the next step in her evaluation?
1) repeat the chest x-ray in six months
2) ultrasound of the left hemithorax
3) bone scan
4) CT scan of the chest
5) proceed directly to bronchoscopy with biopsy