Polymerase chain reaction (PCR) is used widely for pathogen detection. Which modification enables quantification of pathogen load (e.g., viral load in HIV, HBV, HCV) and is the basis of quantitative PCR?

• A Nested PCR — two sequential amplification rounds with internal primers
• B Reverse transcriptase PCR (RT-PCR) — converts RNA to cDNA for amplification
• C Real-time PCR (RT-PCR) using fluorescent dyes/probes (SYBR Green or TaqMan) with continuous fluorescence monitoring during each cycle — product accumulation quantified relative to a standard curve ✓
• D Multiplex PCR — simultaneous amplification of multiple targets

Explanation

Quantitative real-time PCR (qPCR or RT-qPCR) monitors amplification in real time using fluorescent reporters — intercalating dyes (SYBR Green) or sequence-specific probes (TaqMan). Fluorescence intensity increases proportionally with PCR product accumulation. The cycle threshold (Ct) value — the cycle at which fluorescence crosses a threshold — inversely correlates with starting template quantity. Using a standard curve of known concentrations, exact viral/bacterial load is calculated. Note: 'RT-PCR' is an ambiguous abbreviation — it can mean real-time PCR (quantitative) OR reverse-transcriptase PCR (RNA-to-cDNA); context determines meaning.

Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.

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