Which of the following is a direct positive acute-phase reactant?

• A. ESR
• B. Ferritin
• C. CRP
• D. Albumin

Answer: (C)

In inflammation, the liver ramps up production of acute-phase proteins in response to cytokines, especially IL-6. These proteins fall into two groups: positive acute-phase reactants, which increase, and negative ones, which decrease. The protein that rises most rapidly and directly in this response is C-reactive protein. It is synthesized by hepatocytes in reaction to inflammatory signals, reaches measurable levels within hours, and falls quickly as inflammation resolves, making it a sensitive, real-time marker of inflammatory activity. ESR, ferritin, and albumin do not fit as the direct positive acute-phase reactant in this context: ESR is a sedimentation test influenced by several factors rather than a direct inflammatory protein; ferritin can rise with inflammation but is also tied to iron storage and other conditions, making it less specific; albumin decreases during inflammation as a negative acute-phase protein, not a positive one. Therefore, CRP best fits the description of a direct positive acute-phase reactant.