**Core Concept**
Anaphylaxis is a severe, life-threatening allergic reaction that requires immediate medical attention. The presentation includes symptoms such as nausea, vomiting, abdominal cramps, flushing, tachycardia, bronchospasm, and hypotension.

**Why the Correct Answer is Right**
The first priority in managing an episode of anaphylaxis is to administer epinephrine (adrenaline) via an auto-injector or intramuscular injection. Epinephrine works by binding to beta-2 adrenergic receptors in the bronchi, causing bronchodilation and relieving bronchospasm. Additionally, epinephrine increases heart rate and blood pressure, which helps to counteract the hypotension associated with anaphylaxis. The administration of epinephrine is crucial in the initial management of anaphylaxis, as it can help to prevent progression to a more severe and potentially fatal outcome.

**Why Each Wrong Option is Incorrect**
**Option A:** Administering antihistamines such as diphenhydramine is not the first priority in managing anaphylaxis. While antihistamines can provide some relief from symptoms, they do not address the underlying pathophysiology of anaphylaxis and may not be effective in preventing progression to a severe outcome.

**Option B:** Calling the emergency services is an important step in managing anaphylaxis, but it is not the first priority. The administration of epinephrine is the first priority, as it can help to stabilize the patient and prevent progression to a more severe outcome.

**Option C:** Administering oxygen is not the first priority in managing anaphylaxis. While oxygen may be necessary in some cases, the administration of epinephrine is the first priority.

**Option D:** Administering corticosteroids is not the first priority in managing anaphylaxis. Corticosteroids may be used to prevent the late phase of an allergic reaction, but they do not address the immediate need for epinephrine in managing anaphylaxis.

**Clinical Pearl / High-Yield Fact**
In the event of anaphylaxis, the administration of epinephrine should be followed by the administration of antihistamines and corticosteroids to provide additional relief from symptoms and prevent the late phase of the allergic reaction.

**Correct Answer:** C. Administer epinephrine via an auto-injector or intramuscular injection.