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When a test requires SERUM (not plasma) and rapid turnaround is critical, a thrombin-based rapid serum tube is appropriate—IF your laboratory has validated it for the requested analyte panel. Thrombin accelerates clot formation, allowing centrifugation within minutes and preserving the required matrix. Option C aligns with this approach. Waiting the full clot time in an SST (Option A) delays care, and anticoagulant therapy may extend clotting beyond 30 minutes. Substituting plasma (Option B) may be unacceptable if the assay is serum-validated only; matrix substitution without authorization can produce biased results. EDTA (Option D) chelates calcium and interferes with many chemistry assays (e.g., falsely low calcium, altered enzyme activities) and is not an acceptable serum substitute. Always confirm local validation, label tubes correctly, mix by gentle inversion, and document collection/separation times. Understanding matrix requirements and validated alternatives is a hallmark of strong preanalytical practice and a frequent exam theme.