**Core Concept**
The first and most important measure in the management of severely injured patients is to ensure adequate airway, breathing, and circulation (ABC) through the primary survey, which is the foundation of the Advanced Trauma Life Support (ATLS) protocol.

**Why the Correct Answer is Right**
The primary survey, also known as the ABCDE approach, is a systematic method of assessing and managing critically injured patients. It involves checking the patient's airway, breathing, circulation, disability (neurological status), and exposure (for any other injuries). This approach helps to identify life-threatening injuries early and prioritize treatment accordingly. The primary survey is a critical component of trauma care, as it enables healthcare providers to rapidly assess and manage the most severe injuries, thereby improving patient outcomes.

**Why Each Wrong Option is Incorrect**
**Option A:** While fluid resuscitation is an essential part of trauma care, it is not the first measure in managing severely injured patients.
**Option B:** Although imaging studies, such as X-rays or CT scans, are crucial in diagnosing injuries, they are not the initial step in managing severely injured patients.
**Option C:** While pain management is an important aspect of trauma care, it is not the first priority in managing severely injured patients.

**Clinical Pearl / High-Yield Fact**
The primary survey should be performed in a stepwise manner, with each component checked before moving on to the next. This approach helps to ensure that no life-threatening injuries are missed, and treatment can be prioritized accordingly.

**Correct Answer: C. Primary survey (ABCDE approach)**