A patient post-emesis of dark red blood shows tachycardia but maintains a low normal blood pressure. What is the primary treatment priority?

The primary treatment priority in this scenario should focus on addressing the potential hypovolemic shock caused by blood loss from the emesis of dark red blood. The symptoms of tachycardia indicate that the body's compensatory mechanisms are working to maintain circulation despite a potential decrease in blood volume. Administering oxygen is considered a critical step to support the patient's oxygenation, particularly if they are showing signs of distress or decreased perfusion.

By providing supplemental oxygen, the aim is to enhance tissue oxygenation, especially in the setting where the patient may be experiencing decreased circulatory volume. This is particularly important in a condition where blood loss could compromise the delivery of oxygen to vital organs.

While administering IV fluids is also crucial for volume resuscitation and would typically follow closely behind oxygen administration, the immediate priority in this context is to stabilize the patient's oxygenation, thereby reducing the risk of further complications. The other options, such as starting a blood transfusion or obtaining laboratory results like a CBC, are indeed important next steps but should follow the immediate supportive and stabilizing measures such as oxygen therapy.