An elderly gentleman, aged 80 years, arrives at the ER by ambulance transport. His visiting nurse found him febrile, disoriented, and with labored breathing. His cough was productive of a rusty-yellow sputum. At the hospital, CXR (chest X-ray) shows focal lobar infiltrates. A gram stain of his sputum contained gram-positive diplococci. Capsule-specific antibodies bound to the diplococci resulted in a positive Quellung reaction. Which is the most likely pathogen for this community-acquired pneumonia?