Hypothesis 3
Infection of fetlock joint
There signs of inflammation would be present, heat, swelling, pain, lack of use
Bacterial toxins are causing the inflamation
Neutrophils will arrive and further the process of inflammation
The cellular response involves neutrophils and macrophages.  The function of these leukocytes is to defend area against infecton, removal of necrotic debris as well as initiating repair.
If the infection is spread via a hematogenous route multiple areas of inflammation would be expected. 
On physical exam no puncture wounds were seen, however if the infection was not due to hematogenous spread then a puncture wound is the most likely cause.
May progress to septic arthritis
Pain associated with the inflammatory mediators will result in a lameness grade 3/4.