A 28-year-old man who is a singer/songwriter has been experiencing hard times for the past 3 years. He has played at a couple of clubs a night to earn enough to avoid homelessness. He comes to the free clinic because he has noticed that his voice quality has become progressively hoarser over the past year. On physical examination, he is afebrile. There are no palpable masses in the head and neck area. He does not have a cough or significant sputum production, but he has been advised on previous visits to give up smoking. Which of the following is most likely to produce these findings?