Acute Bronchitis: This is the most likely diagnosis given the patient's symptoms of cough with purulent sputum. Acute bronchitis is a common condition characterized by inflammation of the bronchial tubes, often caused by a viral or bacterial infection. The presence of purulent sputum suggests a possible bacterial component.

Pneumonia: This is a plausible diagnosis, especially if the patient has a fever, chest pain, or difficulty breathing, in addition to the cough and purulent sputum. Pneumonia can be caused by bacteria, viruses, or fungi and requires prompt treatment.
Chronic Obstructive Pulmonary Disease (COPD) Exacerbation: If the patient has a history of COPD, an exacerbation could present with increased cough and purulent sputum production. This condition is characterized by chronic inflammation and damage to the lung tissue.
Asthma: Although less common in a new diagnosis at the age of 55, asthma can present with cough and, occasionally, purulent sputum if there is a secondary infection. However, asthma typically includes symptoms of wheezing and shortness of breath.

Lung Cancer: Although less likely, lung cancer can present with cough and purulent sputum, especially if there is an obstructing tumor causing post-obstructive pneumonia. Given the potential severity of this diagnosis, it's crucial not to miss it, especially in a patient with a history of smoking.
Tuberculosis (TB): TB can present with chronic cough, sometimes producing purulent sputum. It's essential to consider TB, especially if the patient has been exposed to someone with TB, has a weakened immune system, or has recently traveled to an area with high TB prevalence.
Pulmonary Embolism: While pulmonary embolism typically presents with sudden onset of dyspnea and chest pain, in some cases, it can cause cough. It's a critical diagnosis not to miss due to its high mortality rate if untreated.

Cystic Fibrosis: Although rare in a new diagnosis at the age of 55, cystic fibrosis can present with recurrent respiratory infections, including cough with purulent sputum. It's more commonly diagnosed in childhood, but some cases may not be diagnosed until adulthood.
Bronchiectasis: This condition involves permanent enlargement of parts of the airways and can lead to chronic cough and purulent sputum production. It can be primary or secondary to various conditions, including infections, autoimmune diseases, or genetic disorders.
Aspiration Pneumonia: If the patient has a history of dysphagia or other conditions leading to aspiration, aspiration pneumonia could be a rare but possible diagnosis, presenting with cough and possibly purulent sputum if a secondary infection develops.