Drug Eruptions: Morbilliform and Fixed Drug

Cutaneous adverse drug reactions (CADRs) affect 2-3% of hospitalized patients and represent a spectrum from benign morbilliform eruptions to life-threatening conditions (SJS/TEN, DRESS). The NP must differentiate these patterns, identify the causative agent, and determine which reactions are self-limited versus which require emergent intervention. Morbilliform (exanthematous) drug eruption: The MOST COMMON drug eruption pattern (~95% of all drug rashes). Pathophysiology involves a delayed-type (Type IVb) T-cell-mediated hypersensitivity reaction. Drug molecules or their metabolites are presented by antigen-presenting cells (Langerhans cells) to CD4+ T-helper cells via MHC class II molecules, triggering release of IL-5 and other Th2 cytokines that recruit eosinophils to the dermis. The reaction typically develops 7-14 days after FIRST exposure to the drug (or 1-3 days upon RE-EXPOSURE). Clinical presentation: symmetric, erythematous macules and papules starting on the trunk and spreading centrifugally to the extremities, often becoming confluent; pruritus is common; mucous membranes are SPARED (important distinction from SJS/TEN). The eruption is typically self-limited, resolving within 1-2 weeks of drug discontinuation, often with desquamation. Most common causative drugs: aminopenicillins (amoxicillin — especially during EBV infection/mononucleosis, where the rash...