All of the following organisms cause a clinical effect via the production of an exotoxin except:

The correct answer is C. Escherichia coli.

Escherichia coli (E. coli) can cause clinical effects through various mechanisms, including:

– Endotoxin production (lipopolysaccharides)
– Adhesion and colonization
– Invasion of host cells
– Production of cytotoxins (e.g., Shiga toxin)