An agricultural worker from the San Joaquin Valley presents with chronic, indolent granulomatous osteomyelitis of the distal radius. Radiographs show lytic lesions with minimal reactive sclerosis. Histological biopsy reveals distinct thick-walled spherules containing endospores. Which of the following is the causative organism?

Explanation:
Coccidioidomycosis (Valley Fever) is an endemic fungal infection native to the southwestern United States (such as the San Joaquin Valley) and parts of Latin America. The fungus, Coccidioides immitis, can disseminate to bone, causing lytic lesions typically without surrounding sclerosis. The pathognomonic histological finding is the presence of large, thick-walled spherules filled with endospores within a granulomatous inflammatory background. Reference: Campbell's Operative Orthopaedics, 14th Edition.