A pregnancy test produces a visible test line only when sufficient labelled antibody–hCG complexes are trapped there. A urine sample contains hCG at a concentration below the detection limit. The control line appears but the test line does not.
Which explanation is correct?
Too few labelled complexes accumulate to produce a visible test line.
No labelled antibodies move because the urine contains some hCG molecules.
All labelled antibodies bind to the control line before reaching hCG.
Too many labelled complexes accumulate and prevent a visible test line.
45 exam-style questions on Edexcel GCSE Biology 5.3 Immune system and medicines, covering 5.3.1 The specific immune system, 5.3.2 Response to immunisation, 5.3.3 Advantages and disadvantages of immunisation, 5.3.4 Antibiotics and bacterial infections, 5.3.5 Aseptic techniques in culturing, 5.3.6 Cross-sectional areas of cultures, 5.3.7 Stages of developing new medicines, and 5.3.8 Production of monoclonal antibodies. Each one has a worked solution and a mark scheme showing where the marks go.