A patient with no significant medical history presents with a three-day history of pleuritic chest pain, chills, and a productive cough. On examination, dullness to percussion and egophony are noted at the right lung base. The patient's oxygen saturation is 92% on room air. Which organism is most likely the causative agent?
Klebsiella pneumoniae
Haemophilus influenzae
Legionella pneumophila
Streptococcus pneumoniae