In the VDRL test, which component serves as the antigen?

• A. Inactivated Treponema pallidum
• B. Cardiolipin
• C. Lecithin
• D. Cholesterol

Answer: (B)

In this test, the antibodies being detected are anti-lipid antibodies (reagins) that target cardiolipin. The antigen used is a cardiolipin-containing complex formed with lecithin and cholesterol. Because cardiolipin is the lipid component that these antibodies most readily recognize, it serves as the antigen in the VDRL reaction. Lecithin and cholesterol help create the proper lipid surface for the reaction, but the key reactant that the antibodies bind to is cardiolipin. This is why the test is non-treponemal and why cardiolipin is the correct antigen. Inactivated Treponema pallidum would be used in treponemal-specific tests, not VDRL, reinforcing that the cardiolipin component is the essential antigen here.