Final answer:
Option (A), To control significant external bleeding where direct pressure is ineffective and a tourniquet cannot be used, the best option is to use a topical hemostatic agent or apply a pressure bandage; never discontinue direct pressure until the bleeding is under control.
Step-by-step explanation:
When you are attempting to control significant external bleeding and direct pressure alone is ineffective, and the injury is in a location that does not allow the use of a tourniquet, the best course of action would be to use a topical hemostatic agent. These agents typically contain materials that rapidly accelerate the body's natural coagulation process and are effective in controlling bleeding when direct pressure is not enough. When this option is not available or suitable, applying a pressure bandage can also be beneficial as it maintains pressure on the wound to help control bleeding.
In the scenario provided, one must never discontinue direct pressure when trying to control hemorrhage until further steps to control the bleeding have been established as it could exacerbate blood loss. Also, placing a patient in a seated position is generally not advisable, as it depends on the location and nature of the injury and could potentially worsen the bleeding or lead to shock.
Platelets and substances released by the damaged vessel wall signal for more platelets to accumulate, which in turn release more chemicals attracting even more platelets and accelerating clot formation. This is an example of a positive feedback mechanism that is critical in the bodily response to hemorrhage and blood loss.