**Core Concept**
Post-traumatic stress disorder (PTSD) is a complex mental health condition that develops in individuals who have experienced or witnessed a traumatic event. The disorder is characterized by a range of symptoms, including flashbacks, nightmares, severe anxiety, and uncontrollable thoughts about the event.

**Why the Correct Answer is Right**
PTSD symptoms can be broadly categorized into four clusters: re-experiencing, avoidance, negative alterations in cognition and mood, and alterations in arousal and reactivity. These symptoms are often accompanied by significant distress or impairment in social, occupational, or other areas of functioning. The clinical features of PTSD include flashbacks, nightmares, severe anxiety, and uncontrollable thoughts about the event.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because flashbacks are a common clinical feature of PTSD.
**Option B:** This option is incorrect because severe anxiety is a characteristic symptom of PTSD.
**Option C:** This option is incorrect because uncontrollable thoughts about the event are a hallmark of PTSD.

**Clinical Pearl / High-Yield Fact**
It is essential to recognize that PTSD can manifest differently in various individuals, and the symptoms may not always be immediately apparent. A comprehensive diagnostic evaluation, including a detailed history and psychological assessment, is necessary to accurately diagnose PTSD.

**Correct Answer:** D.