Final answer:
The most common cause of community-acquired pneumonia in adults is Streptococcus pneumoniae, a gram-positive bacterium that is part of the normal respiratory microbiota.
Step-by-step explanation:
The most common etiologic agent of community-acquired pneumonia in adults is Streptococcus pneumoniae. This gram-positive bacterium is often found as part of the normal microbiota of the human respiratory tract and is the primary cause of bacterial pneumonia. Known for its polysaccharide capsule, Streptococcus pneumoniae hinders phagocytic clearance, leading to an inflammatory response in the lungs with symptoms such as a productive cough with sometimes bloody sputum. While Mycoplasma pneumoniae is known for causing a milder form of the disease often called "walking pneumonia," it is not the most common cause.