Reactive Arthritis - Enthesitis

A 27-year-old man presents with a 4-week history of asymmetric large joint swelling (right knee and left ankle), posterior heel pain worse in the morning, and inflammatory low back pain. He reports a self-limited episode of dysenteric diarrhea with fever 5 weeks ago after eating undercooked chicken while travelling. On examination, he has bilateral conjunctivitis, painless shallow oral ulcers, circinate balanitis, and keratoderma blennorrhagicum. HLA-B27 is positive; stool PCR identified Campylobacter jejuni. A lateral calcaneal radiograph is shown. The structure marked **A** in the diagram demonstrates fluffy irregular periosteal new bone formation at the plantar fascia origin. Which of the following best describes the radiographic hallmark of enthesitis in this condition?