Diagnosis:

Our diagnosis is trauma induced septic arthritis via direct inoculation of the right hind fetlock. 
This is a combination of our first and third hypotheses. As mentioned in the interpretation of specialty exams, we have ruled out fractures or developmental abnormalities. The isolation of a pure culture of Klebsiella from the joint tap indicates infectious arthritis. The history obtained from the owner (suspicion that the mare stepped on the foal) and the fact that only one joint was involved and few systemic signs were noted (no elevation in white cell counts) led us to the diagnosis of direct joint inoculation.
Although hematogenous spread may be a etiological possibility for the infectious arthritis it is unlikely. The foal did not demonstrate the normal predisposing factors such as failure of passive transfer nor does the foal demonstrate multiple joint involvement. Although these facts do not entirely rule out hematogenous spread they do indicate that it is a less likely cause of the infectious arthritis.