**Core Concept**
Post-Traumatic Stress Disorder (PTSD) is a mental health condition that develops after an individual experiences or witnesses a traumatic event. The disorder is characterized by symptoms such as flashbacks, nightmares, and severe anxiety, as well as avoidance of triggers that remind the individual of the traumatic event.

**Why the Correct Answer is Right**
PTSD is a diagnosis based on the presence of specific symptoms that last for more than one month and cause significant distress or impairment in social, occupational, or other areas of functioning. The symptoms of PTSD can be categorized into four main clusters: intrusion (e.g., flashbacks, nightmares), avoidance, negative alterations in cognitions and mood (e.g., guilt, shame), and alterations in arousal and reactivity (e.g., hypervigilance, exaggerated startle response).

**Why Each Wrong Option is Incorrect**
**Option A:** The statement that "PTSD is a result of a weak nervous system" is incorrect because PTSD is not a result of a weak nervous system, but rather a result of a strong stress response to a traumatic event. The nervous system of an individual with PTSD is actually overactive and hyperresponsive to stimuli.

**Option B:** The statement that "PTSD is a type of anxiety disorder" is partially correct, but it is not entirely accurate. While PTSD shares some symptoms with anxiety disorders, it is classified as a distinct trauma- and stressor-related disorder in the Diagnostic and Statistical Manual of Mental Disorders (DSM-5).

**Option C:** The statement that "PTSD can be diagnosed immediately after a traumatic event" is incorrect because PTSD diagnosis requires the presence of symptoms that last for more than one month after the traumatic event. Acute stress disorder (ASD) is a diagnosis that can be made immediately after a traumatic event, but it requires symptoms to last for at least three days but less than one month.

**Option D:** The statement that "PTSD is a chronic condition that cannot be cured" is incorrect because while PTSD is a chronic condition, it can be effectively treated with a combination of psychotherapy and medication. Cognitive-behavioral therapy (CBT), specifically, has been shown to be an effective treatment for PTSD.

**Clinical Pearl / High-Yield Fact**
It is essential to note that PTSD diagnosis requires the presence of symptoms that last for more than one month after the traumatic event. If symptoms resolve within a month, a diagnosis of acute stress disorder (ASD) may be considered.

**Correct Answer:** D.