Lumpy jaw is caused by which organism?

Lumpy jaw, also known as actinomycosis, is primarily caused by Actinomyces israelii, which is a type of bacterium that is part of the normal flora in the human oral cavity. This organism can become pathogenic under certain conditions, particularly when there is a break in the mucosal barrier, often associated with dental procedures or trauma. Actinomyces israelii thrives in anaerobic environments, and infections typically result in abscess formation and the development of painful swellings in the jaw, leading to the characteristic "lumpy" appearance.

In contrast, the other organisms listed are associated with different diseases. Streptococcus pneumoniae typically causes pneumonia, meningitis, and otitis media, while Bordetella pertussis is the causative agent of whooping cough. Nocardia asteroides is associated with nocardiosis, a pulmonary infection that can also disseminate to other regions of the body. Thus, the distinctive nature of Actinomyces israelii's relationship with lumpy jaw sets it apart as the correct choice.