## Core Concept
Post-traumatic stress disorder (PTSD) is a psychiatric condition that develops after experiencing or witnessing a traumatic event. It is characterized by symptoms such as flashbacks, severe anxiety, and uncontrollable thoughts about the event. The pathophysiology of PTSD involves alterations in brain regions and neurotransmitter systems, including the **amygdala**, **hippocampus**, and **stress response systems**.

## Why the Correct Answer is Right
The correct answer involves understanding which option is not associated with PTSD. While options A, B, and C are linked to PTSD through various studies and clinical findings, option D, **Cushing's syndrome**, is not directly associated with PTSD. Cushing's syndrome is a hormonal disorder caused by prolonged exposure of the body's tissues to high levels of **cortisol**. Although individuals with PTSD may have altered cortisol levels, Cushing's syndrome is a distinct medical condition and not a characteristic or direct consequence of PTSD.

## Why Each Wrong Option is Incorrect
- **Option A:** Hypervigilance is a core symptom of PTSD. Individuals with PTSD often exhibit an exaggerated startle response and are constantly on guard, expecting a threat.
- **Option B:** Intrusive memories or flashbacks are hallmark symptoms of PTSD, reflecting the re-experiencing symptoms that are a key component of the disorder.
- **Option C:** Avoidance behaviors are also characteristic of PTSD, where individuals go to great lengths to avoid thoughts, feelings, or situations that remind them of the traumatic event.

## Clinical Pearl / High-Yield Fact
A key clinical pearl for PTSD is the importance of early intervention and the use of **cognitive-behavioral therapy (CBT)**, particularly **prolonged exposure therapy**, as a first-line treatment. Additionally, medications such as **SSRIs** (selective serotonin reuptake inhibitors) are commonly used to manage symptoms.

## Correct Answer Line
**Correct Answer: D. Cushing's syndrome**