Mrs. Z is 55 yeаrs оld аnd cоmes tо see you for continued productive cough. She complаins of "coughing clear stuff up all day every day" for the last 4 months. She verbalizes frustration that she experiences this every year around the same time for the last few years. She denies fever, headache, nasal congestion, runny nose, post nasal drip, cough, nausea, vomiting, or diarrhea. In your workup, you find no obvious explanation for her cough and her CXR today is normal. Her most likely diagnosis is: