Choice A is wrong because hemoglobin is the protein in red blood cells that carries oxygen. It is not affected by warfarin therapy and does not indicate the risk of bleeding or clotting.

Choice B is wrong because hematocrit is the percentage of blood volume that is occupied by red blood cells. It is also not affected by warfarin therapy and does not indicate the risk of bleeding or clotting.

Choice C is wrong because platelet count is the number of platelets in the blood. Platelets are cell fragments that help with blood clotting by forming a plug at the site of injury. Warfarin does not affect platelet production or function, but rather inhibits the clotting factors that are needed to stabilize the platelet plug.

Choice D is wrong because prothrombin time (PT) is the time it takes for blood to clot in the presence of a specific reagent. It is affected by warfarin therapy, but it is not standardized and may vary depending on the reagent and the laboratory.

The nurse should monitor the INR regularly for a patient who is taking warfarin (Coumadin), which is an anticoagulant that inhibits the synthesis of vitamin K-dependent clotting factors in the liver. The INR is a standardized measure of the prothrombin time (PT), which reflects the time it takes for blood to clot. The INR should be between 2 and 3 for most patients on warfarin therapy, unless otherwise specified by the provider.