The complication least likely to occur in a case of chronic bronchitis is:

Spontaneous pneumothorax is a complication of rupture of a bulla in an emphysematous patient. Chronic hypoxia in chronic bronchitis leads to pulmonary aery hypeension and cor pulmonale. CO2 retention leads to respiratory acidosis. Long term hypoxia leads to increased hemoglobin production leading to polycythemia.