**Core Concept**
The management of a stab injury to the left flank involves a thorough evaluation to rule out internal organ damage. The patient's stable vital signs do not necessarily indicate the absence of significant internal bleeding or other injuries. A prompt and accurate assessment is crucial to prevent further complications.

**Why the Correct Answer is Right**
The next step in management would be to perform a focused assessment with sonography for trauma (FAST) to quickly evaluate the patient's peritoneal cavity for any evidence of bleeding or fluid accumulation. This bedside ultrasound examination can help identify free intraperitoneal fluid, which may indicate internal organ damage or hemorrhage. The FAST exam is particularly useful in this scenario as it is non-invasive, quick, and can be performed concurrently with other resuscitative efforts.

**Why Each Wrong Option is Incorrect**
**Option A:** Immediate surgical exploration is not the next step without first confirming the presence of significant internal bleeding or organ damage. This approach could lead to unnecessary surgical intervention and increased risk of complications.

**Option B:** The patient's stable vital signs do not necessarily indicate the absence of significant internal bleeding or other injuries. A more thorough evaluation is required before considering discharge or further observation.

**Option C:** Chest X-ray is not the next step in managing a stab injury to the left flank, as it may not provide adequate information about the extent of internal damage or bleeding.

**Clinical Pearl / High-Yield Fact**
The American College of Surgeons Committee on Trauma (ACS COT) recommends a standardized approach to trauma care, which includes the use of FAST exam as a rapid assessment tool to identify patients at high risk of significant injury.

**Correct Answer:** C. Chest X-ray