**Core Concept**
Post-traumatic stress disorder (PTSD) is characterized by a core symptom of re-experiencing the traumatic event, which distinguishes it from other anxiety disorders like generalized anxiety disorder or panic disorder. This re-experiencing is a hallmark of PTSD and is rooted in maladaptive memory consolidation and emotional dysregulation following trauma.

**Why the Correct Answer is Right**
Re-living the traumatic event—such as intrusive memories, flashbacks, or distressing dreams—is a defining feature of PTSD according to the DSM-5. This symptom is not typically present in other anxiety disorders. While nightmares, hypervigilance, and avoidance are common in PTSD, re-experiencing is the most specific and diagnostic criterion that differentiates it from other anxiety conditions. It reflects a pathological reactivation of trauma-related memories.

**Why Each Wrong Option is Incorrect**
Option A: Nightmares occur in PTSD but are also seen in other conditions like sleep disorders or depression, making them insufficient for differentiation.
Option C: Hypervigilance is a PTSD symptom but is also observed in other anxiety and chronic stress states, such as in generalized anxiety disorder or adjustment disorder.
Option D: Avoidance behavior is common in both PTSD and other anxiety disorders, such as social anxiety or phobias, and thus lacks specificity.

**Clinical Pearl / High-Yield Fact**
The **"re-experiencing"** symptom is the most specific diagnostic criterion for PTSD and must be present for at least one month to meet diagnostic thresholds. This symptom is not reliably seen in other anxiety disorders and is central to the DSM-5 diagnostic criteria.

✓ Correct Answer: B. Re-living of past adverse event