What is the causative agent of a gram-negative coccobacillus that can lead to vertebral osteomyelitis or septic arthritis?

Brucellosis, caused by the Brucella species, is indeed a known causative agent of vertebral osteomyelitis and septic arthritis, particularly in individuals with a history of exposure to infected animals or consumption of unpasteurized dairy products. The Brucella bacteria are gram-negative coccobacilli and are capable of causing various systemic infections, including musculoskeletal complications.

In cases of brucellosis, patients often present with fever, sweats, malaise, and possibly joint involvement, where the bacteria can lead to inflammation of joints or the spine due to hematogenous spread. The association between Brucella and these specific complications is well documented, making it the correct answer here.

Other options may involve different types of infections or present different clinical profiles. Although Tularemia, Haemophilus influenzae, and Legionella can also lead to serious infections, their typical presentations and associations differ significantly from those of Brucella, especially concerning osteomyelitis or septic arthritis.