Which factor is least likely to increase the affinity of hemoglobin for oxygen?

Hemoglobin's affinity for oxygen is influenced by several factors, including temperature, pH, and carbon dioxide levels, often summarized by the Bohr effect. When the temperature increases, the affinity of hemoglobin for oxygen decreases. This is because higher temperatures promote the release of oxygen from hemoglobin, which is a part of the physiological response to increase oxygen delivery to actively metabolizing tissues that are generating heat.

In contrast, higher pH (alkalosis) and lower levels of carbon dioxide enhance hemoglobin's affinity for oxygen, as they shift the equilibrium towards promoting oxygen binding. Similarly, lower temperatures also increase hemoglobin's oxygen affinity. These conditions stabilize the R state (relaxed state) of hemoglobin, which favors oxygen binding.

Thus, when evaluating the factors affecting hemoglobin's oxygen affinity, higher temperature is indeed the one least likely to increase affinity, as it actually reduces it instead.