**Core Concept**
The first step in CPR involves recognizing the need for immediate action to restore blood circulation and breathing in a person who has stopped breathing or whose heart has stopped beating. This initial step is crucial in preventing further brain damage and cardiac arrest.

**Why the Correct Answer is Right**
The correct answer is to call for medical help, specifically activating the emergency response system by dialing the local emergency number. This step is essential to ensure that trained medical professionals, such as paramedics or emergency medical technicians (EMTs), are dispatched to the scene to provide advanced life support and transport the patient to a hospital for further treatment. The American Heart Association (AHA) recommends that bystanders initiate CPR only if they are trained to do so, as improper technique can cause more harm than good.

**Why Each Wrong Option is Incorrect**
**Option A:** Starting CPR without first calling for medical help can lead to exhaustion and decreased effectiveness of the CPR, as well as increased risk of injury to the rescuer.
**Option B:** Checking the patient's airway, breathing, and circulation (ABCs) is an essential step in CPR, but it should be done after calling for medical help and before beginning chest compressions.
**Option C:** Using a defibrillator to shock the heart is a critical step in CPR, but it should be done only by trained medical professionals and after the patient's heart has been determined to be in a life-threatening rhythm.

**Clinical Pearl / High-Yield Fact**
The key to effective CPR is to remember the chain of survival: early recognition of cardiac arrest, early call for help, early CPR, early defibrillation, and early advanced life support.

**Correct Answer: A. Call for medical help.**