**Core Concept**
Post-Traumatic Stress Disorder (PTSD) is a complex psychological condition that develops following exposure to traumatic events, such as combat, abuse, or natural disasters. The disorder is characterized by persistent and intrusive memories, flashbacks, and avoidance behaviors.

**Why the Correct Answer is Right**
PTSD typically involves symptoms such as flashbacks, nightmares, and avoidance of triggers that remind the individual of the traumatic event. The disorder also often includes symptoms of increased arousal, such as difficulty sleeping and irritability. In contrast, **Option A: Dissociation** is not a characteristic feature of PTSD. Dissociation refers to a disconnection from one's thoughts, feelings, or experiences, which is more commonly associated with disorders such as Dissociative Identity Disorder (DID).

**Why Each Wrong Option is Incorrect**
* **Option B:** Hypervigilance is actually a characteristic feature of PTSD, as individuals often experience increased arousal and are constantly "on the lookout" for potential threats.
* **Option C:** Avoidance behaviors are a hallmark of PTSD, as individuals often try to avoid triggers that remind them of the traumatic event.
* **Option D:** Intrusive memories and flashbacks are also characteristic features of PTSD, as individuals often experience unwanted and distressing memories of the traumatic event.

**Clinical Pearl / High-Yield Fact**
It's essential to note that PTSD can manifest differently in different individuals, and not all individuals with PTSD will exhibit all of the characteristic symptoms. A thorough diagnostic evaluation is necessary to accurately diagnose PTSD.

**Correct Answer: D. Intrusive memories and flashbacks are characteristic features of PTSD.**