A 32-year-old woman with systemic lupus erythematosus (SLE) presents to the emergency department with acute onset fever (39.2°C), severe abdominal pain, and bloody diarrhea 3 days after starting sulfasalazine for joint symptoms. Physical examination reveals diffuse abdominal tenderness and hyperactive bowel sounds. Laboratory investigations show: WBC 8,200/μL, hemoglobin 11.2 g/dL, platelets 180,000/μL, and stool culture is negative for pathogens. Colonoscopy reveals mucosal ulceration and inflammation. The patient denies recent antibiotic use or sick contacts. Which type of adverse drug reaction best classifies this presentation?

Explanation

## Classification of This Adverse Drug Reaction ### Clinical Presentation Analysis The patient develops acute, severe gastrointestinal toxicity (hemorrhagic colitis with fever and systemic symptoms) 3 days after sulfasalazine initiation. This is NOT a dose-dependent exaggeration of the drug's expected pharmacological action. ### Type B (Idiosyncratic) Reaction Characteristics **Key Point:** Type B reactions are bizarre, non-dose-dependent, unpredictable adverse effects that occur in genetically susceptible individuals and are NOT related to the drug's known pharmacological properties. | Feature | Type A | Type B | |---------|--------|--------| | **Dose-dependent** | Yes | No | | **Predictable** | Yes | No | | **Incidence** | Common (10–20%) | Rare (1–0.01%) | | **Mechanism** | Exaggerated pharmacology | Genetic/immunological | | **Onset** | Dose-related | Variable, often early | | **Example** | Hypoglycemia from insulin | Drug-induced lupus, Stevens-Johnson syndrome | ### Why This Is Type B 1. **Unpredictable:** Not all patients on sulfasalazine develop hemorrhagic colitis; it occurs in susceptible individuals only. 2. **Non-dose-dependent:** The reaction is not proportional to the dose—it occurs at standard therapeutic doses. 3. **Immunological basis:** Sulfasalazine-induced colitis is mediated by immune hypersensitivity (likely Type IV cell-mediated or immune complex mechanisms), not an exaggeration of its anti-inflammatory effect. 4. **Early onset:** Type B reactions often occur within days to weeks of drug exposure, as seen here (day 3). **High-Yield:** Sulfasalazine is a classic cause of Type B reactions: drug-induced lupus, hemolytic anemia, agranulocytosis, and idiosyncratic colitis. ### Differential Type Classification **Type A (Augmented):** Would manifest as dose-dependent GI upset (nausea, mild diarrhea) seen in most patients—not acute hemorrhagic colitis. **Type C (Chronic):** Develops over prolonged exposure (months to years), e.g., methotrexate-induced cirrhosis or cumulative drug toxicity. **Type D (Delayed):** Teratogenic or carcinogenic effects appearing months to years later, e.g., thalidomide-induced limb defects or chemotherapy-induced secondary malignancy. **Clinical Pearl:** Always consider Type B reactions when a patient develops an unusual, severe, or unexpected adverse effect early in therapy that is not explained by the drug's known pharmacology.