**Core Concept**
The patient's symptoms are characteristic of a sudden onset anxiety disorder, specifically a panic attack, which is a discrete episode of intense fear or discomfort accompanied by physical and cognitive symptoms. Panic attacks are often triggered by specific stimuli or can occur unexpectedly, and they can be a manifestation of various underlying anxiety disorders.

**Why the Correct Answer is Right**
Panic attacks are typically characterized by a sudden onset of symptoms such as palpitations, sweating, trembling, shortness of breath, feeling of choking, chest discomfort, nausea, dizziness, derealization, depersonalization, fear of losing control, fear of dying, and a feeling of impending doom. In this case, the patient's symptoms of sudden onset breathlessness, anxiety, palpitation, and feeling of impending doom are classic features of a panic attack. The normal physical examination suggests that there is no underlying medical condition that could explain these symptoms.

**Why Each Wrong Option is Incorrect**
**Option B:** PTSD (Post-Traumatic Stress Disorder) typically involves symptoms that develop after a person experiences a traumatic event, such as flashbacks, nightmares, severe anxiety, and uncontrollable thoughts about the event. The patient's symptoms do not suggest a traumatic event as a trigger.

**Option C:** Conversion disorder, also known as functional neurological symptom disorder, involves symptoms that affect the nervous system, such as weakness, paralysis, difficulty speaking, or seizures, which are not explained by a medical condition. The patient's symptoms are not indicative of a neurological disorder.

**Option D:** Acute psychosis involves a disconnection from reality, characterized by hallucinations, delusions, disorganized thinking, and abnormal behavior. The patient's symptoms do not suggest a disconnection from reality.

**Clinical Pearl / High-Yield Fact**
Panic attacks can be triggered by various factors, including genetic predisposition, stress, anxiety disorders, and certain medical conditions. It is essential to differentiate panic attacks from other anxiety disorders and medical conditions to provide appropriate treatment.

**✓ Correct Answer: A. Panic attack**