The correct answer is herpes zoster (choice “d”; see discussion). Yeast infections (choice “a”) would be quite unlikely to present with hemorrhagic discrete lesions and far more likely to present with a confluent rash. MRSA (choice “b”) can present in a wide variety of forms but is unlikely to form discrete blisters and would almost certainly involve induration in the area. Without a definitive viral culture, herpes simplex (choice “c”) could not be ruled out in this case, since it presents in a similar manner. But given the patient’s age and the morphology and location of the lesions, zoster was far more likely. See the discussion for further differentiating factors.
The appearance of intralesional hemorrhage in an inflammatory lesion bespeaks a deeper, more vigorous process, since vasculature ends just below the dermoepidermal junction. This occurs infrequently enough with herpes zoster to obscure the diagnosis. Ironically, herpes simplex almost never exhibits this phenomenon, making it useful as a differentiating feature.
In this patient’s case, it seems reasonable to blame the local application of heat as a potential trigger. My experience is that stress is also a factor, though the literature does not bear out that theory.