Which factor can reduce the availability of dNTPs during PCR?

The availability of deoxynucleoside triphosphates (dNTPs) during PCR is critical for successful amplification of DNA. The presence of magnesium ions (Mg2+) plays a pivotal role in this process. Mg2+ is a cofactor that stabilizes the negatively charged dNTPs and is essential for the catalytic activity of DNA polymerases during the PCR. If Mg2+ levels are improperly balanced, it can bind to the dNTPs and potentially lower their effective concentration in the reaction. Therefore, having sufficient Mg2+ is necessary for optimal polymerase activity and dNTP availability.

In contrast, high temperatures can lead to denaturation of the reaction components but do not directly affect dNTP availability. Low pH could adversely impact the stability of nucleotides and enzymes but is less commonly a direct factor in standard PCR protocols, where usually a physiological pH is used. Excessive primer concentration can lead to issues like primer-dimer formations and may affect the reaction efficiency, but it does not inherently reduce the availability of dNTPs. Thus, the presence of Mg2+ is the correct answer as it directly influences the availability and effective utilization of dNTPs in PCR reactions.