One of my very favorite pet peeves is when people refer to medical diseases by incorrect names. "Old Timer's Disease" comes to mind, as does "Lyme's Disease". Well, in this case this patient really does suffer from "Limes Disease" because she is suffering the consequences of chewing on a lime peel, then exposing herself to the sun. The result was first a weird, localized sunburn in the area where the lime juice contacted her skin, followed by this hyperpigmentation at the lateral oral commisure and on her cheek (presumably she rubbed it with her limey fingers). This condition is known as a phytophotodermatitis.
Phytophotos are pretty easy to call, once you know what they are. Anyone who has a localized hyperpigmentation should be questioned about whether or not they had a preceding burn in the area, and if they deny it but the hyperpigmentation is figurate or especially if there are drip marks on the skin, arch your eyebrow, fix them with a steely gaze and say "Really? Would you like to change your answer?" because, of course, they are lying. Or forgetful. Or extraordinarily unobservant. In which case they deserve your abuse, anyway.
There are a lot of plants that have the capacity to cause phytophotos, and limes are among them, but not the only ones. All citrus has the capacity, but limes, lemons and bergamot oranges are among the most common. Also, celery and brussel sprouts (especially from the northeast), figs, and certain wild plants such as some meadow grasses and wild carrots can cause it as well. My pateint who is a grocery guy refers to them as "celery burns" and apparently it is well known to the guys who work with the veggies at Publix.
As a paleodermatologic note, Shalimar perfume used to contain Oil of Bergamot and thus caused many a phytophotodermatitis until they changed the formula. It was known as 'berloque dermatitis"which means "charm (or trinket) dermatitis" because the perfume was applied to the neck below the ears, and so the phytophoto would look like it was in the shape of earrings or hanging charms.
The underlying pathophysiology is that of UVA coming in contact with a furocoumarin derivative, which leads to the initial burn, followed by the hyperpigmentation. The hyperpigmentation is caused by two mechanisms: one, there is pigmentary incontinence (i.e. the pigment leaks from the damaged melanocytes into the superficial dermis after the burn phase) and there is an induction of melanocytes, resulting in the increased number of the pigment-producing cells in the epidermis.
The clue on this patient is the well defined hyperpigmentation in the absence of any clues that made you think it was a nevus, cafe au lait macule or nevus spilus.