**Core Concept:** Post-traumatic stress disorder (PTSD) is a psychiatric disorder that develops in individuals who experience or witness a traumatic event. Clinical features may include re-experiencing the traumatic event, avoidance of stimuli associated with the event, hyperarousal, and alterations in mood and behavior.

**Why the Correct Answer is Right:** The correct answer is not mentioning "hyperarousal" as a clinical feature of PTSD. Hyperarousal refers to the heightened physiological and emotional response to stressors, leading to difficulty in regulating emotions and reactions. Hyperarousal can manifest as irritability, startle response, difficulty sleeping, and increased alertness.

**Why Each Wrong Option is Incorrect:**

A. **Option A:** "Nightmares" is a clinical feature of PTSD, as it represents re-experiencing the traumatic event, which is one of the diagnostic criteria for PTSD.

B. **Option B:** "Irritability" is another clinical feature of PTSD, as it is a common symptom reflecting the heightened emotional state associated with the disorder.

C. **Option C:** "Increased libido" is not a clinical feature of PTSD. In contrast, PTSD can lead to sexual dysfunction, including decreased libido, rather than increased libido.

D. **Option D:** "Feeling cold" is not a clinical feature of PTSD. PTSD can lead to heightened arousal and anxiety, making it less likely to feel cold.

**Clinical Pearl:** PTSD can be challenging to diagnose, as it often presents with overlapping symptoms with other psychiatric disorders. A thorough clinical history and assessment are crucial to identifying the unique features of PTSD, such as re-experiencing, avoidance, and hyperarousal symptoms.