MSFinals-0731

An 82-year-old retired landscape gardener presents to clinic with his daughter, who is concerned about a lesion on his scalp.
The lesion is 45 mm × 30 mm, irregular, and she feels it has changed colour to become darker over recent months. He states it has occasionally bled and is more raised than before. He has a history of travel with the armed forces in South-East Asia.
You suspect malignant melanoma.
Which of the following features is most likely to be associated with a poor prognosis in this patient?