Final answer:
The symptoms and clinical findings described suggest that the smoker is most likely suffering from pneumonia, characterized by lung infection with accumulation of pus and fluids.
Step-by-step explanation:
The most likely diagnosis for a smoker presenting with recent onset shortness of breath, emaciation, moderate respiratory distress, dullness to percussion and egophony at the right lung base, high white blood cells, a chest X-ray showing right lower lobe infiltrate with pleural effusion, and thoracentesis showing purulent, foul-smelling fluid is pneumonia. These symptoms and findings are characteristic of an infection within the lung that leads to fluid and pus accumulation in the alveoli and pleural space, consistent with pneumonia rather than lung cancer, tuberculosis, or pulmonary embolism. Pneumonia is commonly associated with these symptoms and abnormal radiographic findings, which match the presentation of an infection in the lungs caused by various pathogens such as bacteria, viruses, or fungi. Moreover, bacterial infections are often associated with purulent (pus-like) pleural effusions.