A 52-year-old man with hypertension develops acute onset severe headache, fever, and a maculopapular rash 2 weeks after starting a new antihypertensive agent. Laboratory investigations show eosinophilia (8%) and elevated liver enzymes. Which is the most common type of adverse drug reaction responsible for this clinical presentation?

## Classification of Adverse Drug Reactions This clinical presentation — fever, rash, eosinophilia, and hepatitis appearing 2 weeks after drug initiation — is a classic **Type B (Bizarre/Idiosyncratic) reaction**, specifically a **Drug Reaction with Eosinophilia and Systemic Symptoms (DRESS)** or **Drug-Induced Hypersensitivity Syndrome (DIHS)**. ### Type B Reactions: Key Features **Key Point:** Type B reactions are unpredictable, dose-independent, and unrelated to the drug's pharmacological action. They occur in genetically predisposed individuals and involve immune mechanisms. | Feature | Type A | Type B | |---------|--------|--------| | **Dose-dependent** | Yes | No | | **Predictable** | Yes | No | | **Frequency** | Common (80% of ADRs) | Uncommon (15–20%) | | **Mechanism** | Exaggerated pharmacology | Immune/genetic | | **Onset** | Early (dose-related) | Delayed (2–8 weeks) | | **Examples** | Bleeding on warfarin, hypoglycemia on insulin | DRESS, Stevens-Johnson syndrome, anaphylaxis | ### Why This Is Type B 1. **Unpredictable onset** — occurred 2 weeks after initiation, not immediately 2. **Immune-mediated features** — fever, rash, eosinophilia, hepatitis (systemic involvement) 3. **Dose-independent** — the reaction severity is not proportional to drug dose 4. **Genetic predisposition** — certain HLA alleles increase risk (e.g., HLA-A*3101 with carbamazepine) **High-Yield:** DRESS syndrome is a Type B reaction characterized by the triad of **fever + rash + internal organ involvement** (liver, kidney, lymph nodes), often with eosinophilia and atypical lymphocytosis. It is a medical emergency requiring immediate drug withdrawal and often systemic corticosteroids. **Clinical Pearl:** Type B reactions include anaphylaxis, Stevens-Johnson syndrome (SJS), Toxic Epidermal Necrolysis (TEN), DRESS, and serum sickness-like reactions. These are the most serious ADRs and require immediate cessation of the offending drug.