A 12-year-old boy is brought to the office due to a week of nonproductive cough and rash. He has also had low-grade fevers, malaise, and headaches. The patient has no prior medical conditions and is up to date with immunizations. Lung examination reveals bilateral crackles and wheezes. There is a diffuse rash on his torso and extremities as shown in the exhibit.
Which of the following is the most likely cause of this patient's skin lesions?

Question

A 12-year-old boy is brought to the office due to a week of nonproductive cough and rash.  He has also had low-grade fevers, malaise, and headaches.  The patient has no prior medical conditions and is up to date with immunizations.  Lung examination reveals bilateral crackles and wheezes.  There is a diffuse rash on his torso and extremities as shown in the exhibit.  
Which of the following is the most likely cause of this patient's skin lesions?

Quizplus · Accepted Answer

This patient's target-shaped skin lesions likely indicate erythema multiforme (EM), an acute, inflammatory skin disorder that usually develops in the setting of an ongoing infection. Although herpes simplex virus is the most common trigger, respiratory tract infections due to Mycoplasma pneumoniae (eg, low-grade fever, headache, fatigue, cough, wheezes) frequently cause EM in children. EM develops when circulating pathogens are phagocytosed by peripheral mononuclear cells and are brought to the epidermis, where DNA fragments are transferred to keratinocytes via direct cell-to-cell spread (facilited by upregulation of adhesion molecules). Pathogen-specific cytotoxic T-cells then recognize foreign antigens produced by keratinocytes and initiate an inflammatory cascade that results in epithelial damage. Manifestations of EM include target-shaped lesions on the extremities and trunk that have a central dusky area, a surrounding pale ring of edema, and a peripheral halo of erythema. Mucosal lesions also sometimes occur. (Choice B) Erythema nodosum is a delayed-type hypersensitivity reaction that causes inflammatory nodules in the subcutaneous fat. Although it can be triggered by infection, patients generally have tender, erythematous nodules on the bilateral shins (not target-shaped lesions). (Choice C) Psoriasis is an immune-mediated inflammatory disease associated with cytokine-driven keratinocyte hyperproliferation and parakeratosis. It usually causes erythematous, chronic skin plaques with silvery scales on the extensor elbows, scalp, and/or knees. (Choice D) Dermatitis herpetiformis, a manifestation of gluten sensitivity, is caused by the subepidermal deposition of IgA. It is associated with pruritic papules and vesicles on the forearms, knees, and/or scalp (not target-shaped lesions). (Choice E) Urticaria is caused by the release of inflammatory and vasoactive mediators from mast cells in the superficial dermis. Infections (eg, parasites) can cause urticaria, but patients generally have well-circumscribed, raised, pruritic, erythematous plaques. Sometimes the lesions have central pallor, but they are not target-shaped. Educational objective: Erythema multiforme is a target-shaped, inflammatory skin lesion that typically arises in the setting of infection, particularly with herpes simplex virus or Mycoplasma pneumoniae. It is caused by the deposition of infectious antigens in keratinocytes, leading to a strong cell-mediated (eg, cytotoxic T-cell) immune response.

A 12-Year-Old Boy Is Brought to the Office Due to a Week