## **Core Concept**
The patient's presentation of retrosternal chest pain radiating to the left arm, associated with diaphoresis and dyspnea, is highly suggestive of **acute coronary syndrome (ACS)**, specifically **myocardial infarction (MI)**. The immediate goal is to confirm the diagnosis and assess the severity of the condition.

## **Why the Correct Answer is Right**
The most appropriate next investigation in a patient with suspected acute coronary syndrome (ACS) is an **electrocardiogram (ECG)**. An ECG is crucial because it:
- Provides immediate information about the heart's electrical activity.
- Helps diagnose **ST-elevation myocardial infarction (STEMI)**, **non-ST-elevation myocardial infarction (NSTEMI)**, or **unstable angina**.
- Guides immediate management, including the decision for **thrombolysis** or **primary percutaneous coronary intervention (PCI)** in the case of STEMI.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While a **chest X-ray** can provide useful information about the lungs and cardiac silhouette, it is not the immediate next step in diagnosing ACS.
- **Option B:** A **complete blood count (CBC)** may provide supportive information but is not diagnostic for ACS.
- **Option C:** **Troponin levels** are critical for diagnosing MI but take time to result and should not delay immediate ECG evaluation.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that in the setting of ACS, **time is muscle**. Early diagnosis and intervention significantly impact outcomes. The American College of Cardiology (ACC) and American Heart Association (AHA) guidelines emphasize the importance of rapid ECG interpretation in ACS.

## **Correct Answer:** .