Linear IgA Bullous Dermatosis
Don't worry, the cheese isn't slipping off my cracker, but I did want to run this one as a test to see how the retention of the material previously submitted has gone. This is apic of the same patient I previously submitted in mid-January and I will include the discussion below, for a refresher. For the record, the vast majority of you nailed this one, and most noted the "string of pearls".
Here is the previous writeup: Wow! What a tough question to start out the week! I want to point out some clinical features as well as some gamesmanship pointers to help you with questions like this. When looking at an unknown, wither in clinic or in conference, the first question I ask myself is: what is the primary lesion? That will dictate every other thought I devote to the problem. In this particular case, I see blisters in a ring around thrashed epidermis. So, the primary lesion is a blister. Second, do the blisters look fragile or full thickness? In my mind, these look to be full thickness, which immediately rules out pemphigus and its variants. Location? Looks like the inner forearm, which knocks out PCT and pseudoporphyria, since both of those are predominantly on sun damaged skin. So, now we can choose among full thickness blisters like LN2, cantharidin, second degree burn, BP. BP variants, bullous LE, or Linear IgA Bullous Dermatosis. A relatively short, manageable list. It looks way too patterned for BP, BLE, or LN2. And so now we are down to cantharidin, burn or LIGA. Cantharadin is almost always applied only to warts, and the inner forearm is unlikely to develop warts, at least to any significant degree, I told you this is a secondary disease (the patient was treated for something else) and so that rules out burns. What does that leave? Linear IgA. Not only that, but this patient displays the classic "string of pearls" look of blisters in an arc or a ring, which is the most common description of LIGA.
Why not include DH in this DDX? Because the blisters are intact in many cases, which is nearly impossible in a DH patient because they are scratching them to beat the band. Not only that, but they usually look like ant bites. LOTS of PMNs are recruited into the lesions of DH. Additionally, this is an unusual location for DH.
What about impetigo? Well, I gave you the clue that this was a secondary process (see above). Also, no crusting, although the crusting admittedly isn't always there.
What causes LIGA? Almost always, it's Vancomycin. That is always the default answer for LIGA. Other drugs can do it, but always put your money on Vanc.
What is Vanc used for? Pseudomembranous colitis caused by C. difficile, and out of control MRSA in all its protean manifestations (joints, endocarditis, etc.). I would have given points for either answer.
Now, for testmanship: By the way the Q was written, you should assume that the disease the patient was being treated for was something other than what we see. Additionally, the "etiologic agent" part of the Q implies that something that was part of the treatment has something to do with the disease we see. Logically, that would have to be a drug of some sort. Finally, as mentioned above, a lot of answers are eliminated simply by location.
As a final point, when I taught immunobullous diseases at Wilford Hall (the Air Force hospital in San Antonio where I did my residency) I used to joke that it took three hands to do a biopsy on a patient like this: One to hold the punch, one to steady the patient and one to push the biopsying hand away from the primary lesion. The point is, when doing an immunofluorescence biopsy, make sure you biopsy well away from the blisters, preferably at least 1-2 cm away. Otherwise, the inflammation will eat up the IgA you want to display on IF, and as a result you can have a false negative IF.