Chronic Avulsion Fracture with Heterotopic Ossification: This condition occurs when a fragment of bone is pulled away from the main mass of bone due to trauma or stress, leading to the formation of a calcified lump. The 5th tarsometatarsal joint is a common site for such injuries, especially in athletes or individuals with a history of repetitive stress or acute trauma to the area.

Ganglion Cyst with Calcification: Ganglion cysts are benign lumps that can form on the joints or tendons, and while less common, they can become calcified. The location around the 5th tarsometatarsal joint is plausible for a ganglion cyst.
Bursitis with Calcification: Bursae are fluid-filled sacs that cushion bones, tendons, and muscles near joints. Inflammation of a bursa (bursitis) can lead to the formation of a calcified lump, especially if the condition becomes chronic.
Osteochondroma: A type of benign bone tumor that can occur near the ends of bones, where the cartilage forms bone. While not typically presenting as a subcutaneous lump, an osteochondroma near the 5th tarsometatarsal joint could potentially cause a calcified mass to form.

Malignant Bone Tumor (e.g., Osteosarcoma, Chondrosarcoma): Although rare, these conditions can present with a calcified mass. Missing a diagnosis of a malignant tumor could have severe consequences, making it crucial to consider and rule out these conditions with appropriate imaging and potentially a biopsy.
Infection (e.g., Osteomyelitis, Septic Arthritis): Infections of the bone or joint can lead to the formation of a calcified or bony sequestrum as the body tries to wall off the infected area. These conditions require prompt treatment to prevent serious complications.

Tumoral Calcinosis: A rare condition characterized by the formation of calcified masses in the soft tissues around joints, often without an apparent cause. It's more common in certain populations and can be associated with metabolic disorders.
Synovial Chondromatosis: A condition where multiple small cartilaginous nodules form within the synovial membrane of a joint, which can become calcified and potentially extrude into the soft tissues, forming a lump.
Melorheostosis: A rare bone disorder that can cause excessive cortical bone formation, leading to a calcified mass. It typically affects the long bones but can potentially involve any bone, including those in the foot.