**Core Concept**
The diagnostic feature that differentiates Post-Traumatic Stress Disorder (PTSD) from other disorders that occur following a stressful incident is the presence of specific symptoms that are directly related to the traumatic event. PTSD is characterized by a constellation of symptoms, including intrusive memories, avoidance of triggers, and hyperarousal, which are all linked to the traumatic experience.

**Why the Correct Answer is Right**
The correct answer is the presence of symptoms that are directly related to the traumatic event. In PTSD, the symptoms are a result of the individual's inability to process and integrate the traumatic experience into their narrative. This is often referred to as "fear and avoidance" of the traumatic stimuli. The key feature of PTSD is the presence of intrusive memories, flashbacks, and nightmares that are directly linked to the traumatic event. The individual's response to the traumatic event is also a key feature, with symptoms such as hyperarousal, avoidance of triggers, and numbing of emotions. The presence of these symptoms is a result of the individual's attempt to cope with the traumatic experience.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not specify the direct link between the symptoms and the traumatic event. While symptoms such as anxiety and depression can occur after a stressful incident, they are not specific to PTSD.
* **Option B:** This option is incorrect because it does not address the specific symptoms that are directly related to the traumatic event. While avoidance of triggers is a symptom of PTSD, it is not the only symptom and does not specifically address the direct link between the symptoms and the traumatic event.
* **Option C:** This option is incorrect because it does not specify the presence of intrusive memories, flashbacks, and nightmares that are directly linked to the traumatic event. While symptoms such as anxiety and depression can occur after a stressful incident, they are not specific to PTSD.

**Clinical Pearl / High-Yield Fact**
A key feature of PTSD is the presence of symptoms that are directly related to the traumatic event. This can include intrusive memories, flashbacks, and nightmares that are linked to the traumatic experience. It is also important to note that the diagnosis of PTSD requires the presence of symptoms that are severe enough to cause significant distress or impairment in social, occupational, or other areas of functioning.

**Correct Answer:** D.