General Discussion

He may be being treated for a pre-cancerous skin condition

Pre-cancerous actinic keratoses and superficial basal cell carcinomas can be treated with a shortish course of topical creams such as Fluorouracil, usually over 2-4 weeks.

A pal of mine has had thin hair for many years and spent a lot of time outdoors with no hat on, and developed pre-cancerous lesions on his scalp a couple of years ago.

When I saw him after we hadn't met for a while and he'd started the treatment, he looked quite a mess - it temporarily leads to the sort of angry-looking crusty skin patches shown in photos of Trump I've seen. My pal's skin healed just fine. If this is what's going on, Trump's patch seems much more widespread than my pal's were.

The neck area would see a lot of sun over the years, especially when golfing, and the sorts of headgear he's fond of wouldn't have protected that area. I doubt he's been in the habit of using sunscreen (barring the makeup he plasters on in his later years) judging by the state of the rest of his skin.