Nucleic Acids

The question asks for a diagnostic tool capable of detecting very small amounts of a pathogen (bacterium or virus) specifically by amplifying its genetic material (nucleic acid).

Let's analyze the options:

• ELISA (Enzyme-Linked Immunosorbent Assay): This is an immunological technique that typically detects the presence of antigens (proteins from the pathogen) or antibodies (produced by the host immune system in response to the pathogen). While ELISA can be quite sensitive, it does not work by amplifying the pathogen's nucleic acid. Its principle is based on antigen-antibody interactions.
• PCR (Polymerase Chain Reaction): This is a molecular biology technique designed specifically to amplify specific sequences of nucleic acid (DNA, or RNA after a reverse transcription step - RT-PCR). By creating millions or billions of copies of a target DNA/RNA sequence unique to the pathogen, PCR can detect the presence of the pathogen even when it is present in very low concentrations in the original sample. This perfectly matches the description in the question.
• Autoradiography: This technique is used to visualize the location or distribution of radioactive substances within a sample (e.g., on a gel or a tissue section). It is often used as a detection method following other procedures like Southern blotting or in situ hybridization, where a radioactive probe binds to the target nucleic acid. It doesn't inherently amplify the nucleic acid itself to increase sensitivity in the same way PCR does; it detects the signal from labeled molecules.
• r-DNA technology (Recombinant DNA technology): This refers to a broad set of techniques used to join DNA segments from different sources and clone them (e.g., for producing proteins, creating GMOs). While it involves manipulating nucleic acids, it is not primarily a diagnostic technique for detecting low concentrations of pathogens by amplification in the context described.

Therefore, the diagnostic tool that allows the detection of very low concentrations of bacteria or viruses by amplifying their nucleic acid is PCR.