**Core Concept:** In emergency medicine, management of an unresponsive patient with limited history involves a systematic approach to assess vital signs, airway, breathing, and circulation (ABCs).

**Why the Correct Answer is Right:** In this scenario, given that the patient is unresponsive, the primary concern is to ensure their airway, breathing, and circulation are stable before proceeding further. This is why the correct answer is to **Perform a head tilt-chin lift maneuver (ABC step 1)**. This maneuver helps maintain an open airway and facilitate observation of the patient's response to verbal commands and pain stimuli.

**Why Each Wrong Option is Incorrect:**

A. **Perform a chest X-ray (CXR)**: While a CXR may be necessary in some cases, it is not the first step in assessing an unresponsive patient. The ABCs should be addressed first.

B. **Investigate the cause of unresponsiveness first**: This option disregards the importance of ensuring the patient's vital signs and airway are stable before addressing the cause of their unresponsiveness.

C. **Observe the patient without any intervention**: This approach fails to address the patient's immediate needs for a stable airway, breathing, and circulation.

D. **Call for further history**: Although obtaining history is essential, the patient's airway, breathing, and circulation should be stabilized first.

**Why the Correct Answer is Right:** In emergency medicine, the ABCs are crucial for ensuring the patient's survival and preventing further harm. By performing the head tilt-chin lift maneuver, we can maintain an open airway, facilitate breathing, and assess circulation. This allows us to identify and address any life-threatening issues, ensuring the patient's stability before proceeding to further evaluation or obtaining history.

**Clinical Pearl:** In scenarios with limited history or consent issues, prioritizing ABCs can help guide initial management, allowing for better patient care and potentially improving outcomes.