Osteomyelitis: This is the most likely diagnosis given the presentation, as osteomyelitis is an infection of the bone that can present with a variety of symptoms including pain, redness, swelling, and fever. The diagnosis is often suspected based on clinical presentation and confirmed with imaging and laboratory tests.

Cellulitis: An infection of the skin and subcutaneous tissues that can mimic osteomyelitis in its presentation, especially if it is severe and involves the deeper tissues.
Septic Arthritis: An infection within a joint, which can present similarly to osteomyelitis, especially if the infection spreads from the bone to the adjacent joint.
Soft Tissue Abscess: A localized collection of pus in the soft tissues, which can cause symptoms similar to osteomyelitis if it is located near a bone.

Neoplasm (Bone Cancer): Although less likely, bone cancer can present with symptoms similar to osteomyelitis, such as bone pain and swelling. Missing this diagnosis could have severe consequences.
Sickle Cell Crisis: In patients with sickle cell disease, a crisis can cause bone pain that might be confused with osteomyelitis. It's critical to distinguish between the two due to the different management strategies.
Gas Gangrene: A severe infection caused by Clostridium perfringens, characterized by rapid progression and potential for severe morbidity and mortality if not promptly treated.

Chronic Recurrent Multifocal Osteomyelitis (CRMO): A rare condition characterized by recurring episodes of osteomyelitis, often without an identifiable cause. It's more common in children and young adults.
Eosinophilic Granuloma: Part of the spectrum of Langerhans cell histiocytosis, which can cause bone lesions that might be mistaken for osteomyelitis.
Bone Infarct: A condition where there is a loss of blood supply to a bone, leading to bone tissue death. It can present with pain and might be confused with osteomyelitis, especially in patients with risk factors such as sickle cell disease or severe trauma.