A 28-year-old patient has presented to you for the results of their sputum culture analysis. The results report that the sputum in this patient was colonised with Pseudomonas Aeruginosa. This patient was experiencing daily expectoration of large volumes of purulent sputum for several years now with breathlessness in the absence of smoking history. On examination there were coarse crackles in the lower lungs, wheeze and you also noted finger clubbing. What is your main differential diagnosis?