Allergic Contact Dermatitis
Allergic contact dermatitis is either the easiest or the most difficult of diagnoses to make. When you see something that is well-demarcated, linear, or in the shape of an object, think in term of allergic contact. The external stimulus of the immune system is so at odds with the normal "internally generated" rashes we see that it becomes a relatively easy diagnosis. Most of the time, that is. Whether it is from a baseball glove, or soft soaps (the number one contact allergen of the hands in our practice) to nickel dermatitis, there is a pattern to both the rash and the history of onset. Allergic contact dermatitis is considered a Type IV raction and as such is mediated by lymphocytes, which in turn kick up a spongiotic dermatitis, histopathologically. For five points, give me a ddx on spongiotic dermatitis.
On the other hand (pun intended) if you are dealing with dyshidrotic (pompholyx) dermatitis, it tends to prominently involve the palms and the lateral fingers, tends to be more chronic in nature with exacerbations (the history was of an acute episode) and I would have a very, very hard time matching up a stone-cold normal thenar eminence with a pompholyx flare.