What pathogen is likely causing pneumonia and is confirmed by growth on buffered CYE agar with L-cysteine?

Legionella pneumophila is the correct answer because it is known for its association with pneumonia, particularly Legionnaires' disease, and its unique growth requirements. The use of buffered charcoal yeast extract (CYE) agar supplemented with L-cysteine is a specific culture medium that supports the growth of Legionella species, which are difficult to grow on standard media. Legionella requires additional nutrients such as L-cysteine, which is not needed by most other common pathogens responsible for pneumonia.

In contrast, Haemophilus influenzae, Eikenella corrodens, and Streptococcus pneumoniae have different cultivation requirements and do not require the specific conditions provided by CYE agar with L-cysteine. For instance, Haemophilus influenzae typically grows on chocolate agar with factors X and V but does not require L-cysteine. Eikenella corrodens can also grow on standard media and is not dependent on CYE agar. Streptococcus pneumoniae is routinely cultured on blood agar and does not require CYE agar either. Thus, the presence of growth on buffered CYE agar with L-cysteine specifically suggests the presence of Legionella pneumophila, confirming its role as a likely pathogen in this case.