Plantar Fasciitis: This condition is characterized by pain on the bottom of the heel or lateral aspect of the foot, which is exacerbated by weight-bearing activities. The presence of a plantar spur on the X-ray supports this diagnosis, as spurs are often associated with plantar fasciitis. The ineffectiveness of indomethacin, a nonsteroidal anti-inflammatory drug (NSAID), does not rule out this diagnosis, as some cases may require more specific treatments like orthotics, physical therapy, or local injections.

Peroneal Tendinitis: This condition involves inflammation of the peroneal tendons, which run along the lateral aspect of the foot and ankle. It can cause lateral foot pain that worsens with activity, making it a plausible diagnosis given the patient's symptoms.
Lateral Ankle Impingement: This condition occurs when there is compression of the soft tissues in the lateral aspect of the ankle, leading to pain, especially with weight-bearing activities. It could be considered, especially if there's a history of ankle sprains or instability.
Cuboid Syndrome: This involves the cuboid bone in the midfoot and can cause lateral foot pain. It's often associated with activities that involve repetitive foot pronation and supination, which could fit with the exacerbation by weight-bearing activities.

Stress Fracture: Although less common, a stress fracture in one of the bones of the foot could present with localized pain exacerbated by weight-bearing. This diagnosis is critical to consider because, if missed, it could lead to further bone damage or nonunion.
Neoplastic Processes: Tumors, whether benign or malignant, can cause localized pain. Although rare, missing a diagnosis of a neoplastic process could have severe consequences, making it essential to consider, especially if the pain is persistent and unresponsive to typical treatments.
Infection: Osteomyelitis or septic arthritis could present with localized pain and should be considered, especially if there are signs of infection such as redness, warmth, or fever.

Tarsal Tunnel Syndrome: This is a rare condition that involves compression of the posterior tibial nerve as it travels through the tarsal tunnel on the inner side of the ankle. While it typically causes pain and numbness on the inner side of the foot, variant presentations could potentially involve the lateral aspect.
Erythromelalgia: A disorder characterized by burning pain, heat, and redness in the feet, which could potentially be localized to the lateral aspect. It's a rare condition but should be considered in the differential diagnosis of foot pain, especially if other causes have been ruled out.