Test your medicine knowledge with the MKSAP challenge, in partnership with the American College of Physicians.
A 72-year-old man sustains a laceration on his left index finger while preparing chicken. He immediately washes the area and applies neomycin and an occlusive bandage. He changes the bandage and reapplies the medication twice daily. Two days later, he develops itching and redness at the wound site. He has had no fever or other systemic symptoms. Medical history is significant for well-controlled type 2 diabetes mellitus. His only medication is metformin.
On physical examination, vital signs are normal. The left index finger shows a 1.0-cm superficial wound with well-approximated margins without purulence or drainage, and no pain on palpation. There are pinpoint papules and vesicles in an area extending 0.5 cm around the laceration site in a rectangular pattern approximating the bandage. There is no lymphangitic streaking. The remainder of the physical examination is unremarkable.
Which of the following is the most likely diagnosis?
A: Allergic contact dermatitis
B: Group A streptococcal infection
C: Herpes simplex virus infection
D: Staphylococcus aureus infection
MKSAP Answer and Critique
The correct answer is A: Allergic contact dermatitis.
This patient most likely has allergic contact dermatitis from the neomycin. Neomycin is a commonly used over-the-counter topical aminoglycoside antibiotic (often used either alone or as part of combination topical antibiotic preparations). With repeated use, especially on abraded or lacerated skin, neomycin can lead to contact sensitization, which is a T-cell mediated hypersensitivity reaction. Patients and clinicians often mistake this for a wound infection, but if the area is pruritic and there is a geometric, well-defined pattern generally corresponding with the contact area, a contact allergy should be suspected. Discontinuation of the medication and future avoidance are generally recommended.
Group A streptococcal infections would cause skin infections such as impetigo, cellulitis, or erysipelas. None of these infections tend to present with a well-demarcated pattern of involvement, as seen in this patient. This patient has no pain, wound drainage, or discharge that could be associated with bacterial impetigo. There is no pain or lymphangitic streaking typical of cellulitis. Erysipelas would appear as violaceous-red, edematous, well-demarcated plaques on the face or lower extremities, unlike this patient’s presentation.
The classic presentation of herpes simplex virus infection is a group of painful, small vesicles on an erythematous base, transitioning to pustules and subsequent crusting of the lesions over time. Herpetic infection would be painful with no pruritus.
Staphylococcus aureus infection would present with eczematous plaques and open erosions on the flexural folds with pustules in those areas, not in a geometric pattern on a compromised skin barrier typical of allergic contact dermatitis.
- Use of neomycin on abraded or lacerated skin can lead to contact sensitization, which is often mistaken for a wound infection; a contact allergy should be suspected if the area is pruritic and there is a geometric, sharply bordered pattern.
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