Answer lies behind the ears
A rash confined to exposed areas of the skin suggests a photo drug reaction or some other photo-exacerbated disorder.
However, another condition that can give a similar rash in a similar distribution is an airborne contact dermatitis from pollens or dusts.
To differentiate the two, you should look behind the patient’s ears.
This is usually a photo-protected area, so if it is spared then a photo-disorder is more likely than an airborne contact where the allergen can land on any exposed skin, including behind the ear, and cause a reaction.