HISTORY: The patient is a 23 year-old recreational runner who reported acute onset of left foot pain while training for her first marathon. She was on mile 10 of a 16-mile run when she suddenly experienced pain in the region of the medial, plantar aspect of the left mid-foot. There was no specific event that caused the injury, such as stepping on an object or twisting her foot. She was able to finish her run, although the foot pain persisted. The medial mid-foot pain was exacerbated by weight bearing and relieved with non-weight bearing. She provided a history of a leg length discrepancy with the left side long. This had been treated with an insert. She reported no prior foot injuries and no history of stress fractures. There was no numbness, tingling, or weakness, and she reported normal menstrual periods. She had no medical problems and had no surgeries. PHYSICAL EXAMINATION: There was normal foot and ankle alignment. No swelling was present. She was tender to palpation only over the midpoint of the medial longitudinal arch. Active and passive tibiotalar and subtalar motion was normal and did not reproduce any symptoms. There was no pain with motion testing or palpation of the metatarsals, except over the medial longitudinal arch as previously noted. There was normal strength, sensation, and reflexes in the lower limbs. Distal pulses were normal. She did appear to have a leg length discrepancy with the left side 1.5 cm longer. Her gait was antalgic. Pain was reproduced with hopping on the left lower limb. DIFFERENTIAL DIAGNOSIS: 1. Plantar muscle strain 2. Plantar ligament sprain 3. Acute tendon injury 4. Stress fracture TEST AND RESULTS: Left foot AP, lateral, and oblique radiographs: - Normal alignment - No fracture, dislocation, or arthritis Left foot MRI - Partial rupture of the peroneus longus tendon at the distal attachment FINAL WORKING DIAGNOSIS: Partial left peroneus longus tendon rupture at its distal insertion. TREATMENT AND OUTCOMES: 1. Short leg walking boot for four weeks. 2. Physical therapy for a foot and ankle strengthening program. 3. Running gait evaluation. 4. Full, pain-free return to running at eight weeks.