Which statement best defines an immunoassay and explains how antibody–antigen specificity is used diagnostically?

• A. A lab test that uses antibodies that bind specific antigens to detect or measure target molecules in a sample.
• B. A molecular sequencing assay that decodes nucleic acids to detect pathogens by comparing whole genomes to a reference database and then using consensus nucleotide identity thresholds to call positives.
• C. A culture-based method that grows pathogens to detect infection, relying on nonspecific binding of stains and dyes to highlight multiple organisms present in the sample over several days.
• D. A toxicology screen that measures metabolic byproducts indirectly associated with infection by using broadly reactive chemical sensors that respond to enzymatic activity in patient serum samples.

Answer: (A)