A patient with fever, cough, and lobar consolidation on chest X-ray is most likely infected with which Gram-positive diplococcus?

• A. Staphylococcus aureus
• B. Streptococcus pneumoniae
• C. Haemophilus influenzae
• D. Neisseria meningitidis

Answer: (B)

A fever with productive cough and a single lobar consolidation on chest X-ray is classically seen with Streptococcus pneumoniae infection. This organism is a Gram-positive lancet-shaped diplococcus that is the most common cause of community-acquired pneumonia in adults, and it typically produces a dense, well-demarcated consolidation in one lobe. Other organisms listed don’t fit the key clues: Staphylococcus aureus forms Gram-positive cocci in clusters and often causes patchy infiltrates or abscesses, not a clean lobar process; Haemophilus influenzae is a Gram-negative coccobacillus, not Gram-positive; Neisseria meningitidis is a Gram-negative diplococcus and more commonly causes meningitis or septicemia rather than classic lobar pneumonia.