Which organism is most likely responsible for community-acquired lobar pneumonia presenting with fever and lobar consolidation?

• A. Streptococcus pneumoniae
• B. Staphylococcus aureus
• C. Klebsiella pneumoniae
• D. Mycoplasma pneumoniae

Answer: (A)

The main idea is recognizing the organism that most classically causes community-acquired pneumonia with focal, single-lobe consolidation. Streptococcus pneumoniae is the classic culprit. It is a gram-positive, lancet-shaped diplococcus that is alpha-hemolytic and optochin-sensitive. When it causes lobar pneumonia, the infection tends to fill the airspaces of one lobe with protein-rich exudate, producing dense consolidation on imaging.

Clinically, pneumococcal pneumonia often presents with an abrupt high fever, productive cough, pleuritic chest pain, and sometimes rust-colored sputum—features that align with a lobar pattern of consolidation.

In contrast, the other organisms fit different patterns: Staphylococcus aureus often follows influenza and can cause patchy, multilobar involvement or aspiration-type complications; Klebsiella pneumoniae is classically seen in alcoholics with upper-lobe involvement and may cavitate with thick, sometimes currant jelly sputum; Mycoplasma pneumoniae tends to cause an atypical pneumonia with diffuse interstitial infiltrates and less prominent lobar consolidation.

Thus, Streptococcus pneumoniae is the best fit for a community-acquired lobar pneumonia presenting with fever and focal consolidation.