Tarun is a 32 year old man who moved to England from India 6 months ago. He attends his GP practice for a new patient check-up appointment. He confirms that he has no known medical conditions and is fit and well working as an engineer. He says he thinks he had a bad chest infection many years ago which required a several month course of antibiotics, but he cannot recall any other information about the hospital admission.
He admits that he has always had a ‘chesty cough’ and brought up a lot of sputum throughout the day and first noticed this around a year ago. Sometimes he gets breathless after minimal exertion but denies any chest pain, weight loss, fatigue, hoarse voice, fever, night sweats, or haemoptysis.
He has never smoked and there is no family history of note. His observations are stable and you note course bilateral crackles in the lower zones of this chest.
What is his single most likely diagnosis?