What is the most common cause of typical pneumonia?

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

Answer: (D)

The main concept is identifying the most common cause of typical (community-acquired) pneumonia in adults. Streptococcus pneumoniae is the leading culprit because it most frequently colonizes the nasopharynx and, when aspirated into the lower airways, causes the classic lobar consolidation seen in typical pneumonia. It often presents with abrupt onset fever, productive cough with rusty sputum, pleuritic chest pain, and a single affected lobe, which are hallmark features of this pattern.

Other organisms can cause pneumonia too, but they don’t fit the typical pattern as often. Mycoplasma pneumoniae tends to produce an atypical pneumonia with a dry cough and diffuse interstitial infiltrates rather than lobar consolidation. Haemophilus influenzae and Staphylococcus aureus do cause pneumonia, but they are less common overall for the typical presentation in adults (Staph is more linked to post-influenza or hospital-associated cases; Haemophilus is more common in COPD or older adults).

So, Streptococcus pneumoniae is the best answer because it is the most frequent cause of typical, lobar pneumonia in the community.