## Induction Therapy in Moderate-to-Severe Crohn Disease **Key Point:** TNF-α inhibitors (infliximab, adalimumab) are the gold-standard induction agents for moderate-to-severe Crohn disease with active inflammation, particularly when conventional therapy (5-ASA, corticosteroids) has failed or is inadequate. ### Rationale for Infliximab 1. **Mechanism:** Chimeric monoclonal antibody against TNF-α; rapidly reduces inflammatory cytokine signaling in transmural inflammation. 2. **Efficacy:** Induces remission in 60–70% of patients with moderate-to-severe disease within 4–8 weeks. 3. **Transmural disease:** Particularly effective in Crohn disease (which is transmural) compared to ulcerative colitis. 4. **Timing:** Indicated when mesalamine monotherapy and corticosteroids have failed to control disease. ### Why Other Options Are Suboptimal Here | Agent | Role | Why Not First-Line Here | |-------|------|------------------------| | **Mesalamine** | Mild-to-moderate UC; adjunctive in CD | Already failed; insufficient for moderate-to-severe CD | | **Azathioprine** | Steroid-sparing maintenance | Slow onset (8–12 weeks); not for acute induction | | **Budesonide** | Mild-to-moderate ileocolonic CD | Weaker than systemic corticosteroids; not for severe disease | **Clinical Pearl:** The presence of elevated CRP (8.2 mg/dL), bloody diarrhea, and transmural skip lesions on colonoscopy all indicate moderate-to-severe disease requiring biologic therapy, not conventional agents. **High-Yield:** Infliximab is preferred over adalimumab for *induction* in Crohn disease because of faster onset and stronger evidence in ACCENT I trial. Adalimumab is often used for maintenance or in patients with infliximab loss of response.
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