**Core Concept**
The sudden onset of breathlessness, anxiety, palpitations, and a fear of impending doom without any physical examination abnormalities is indicative of a panic attack. Panic attacks are a characteristic feature of panic disorder, a common anxiety disorder that affects a significant portion of the population.

**Why the Correct Answer is Right**
Panic attacks are triggered by the sudden release of stress hormones like adrenaline and cortisol, leading to an exaggerated 'fight or flight' response. This response is mediated by the hypothalamic-pituitary-adrenal (HPA) axis and the sympathetic nervous system. The symptoms of a panic attack, such as rapid heartbeat, shortness of breath, and feelings of impending doom, are a result of the body's attempt to prepare for a perceived threat. In the absence of any physical examination abnormalities, the diagnosis of a panic attack is based on the patient's self-reported symptoms and clinical presentation.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the symptoms described do not typically indicate a cardiac arrhythmia. While palpitations can be a symptom of cardiac arrhythmias, the presence of other symptoms such as breathlessness and anxiety suggests a psychological rather than a cardiac origin.

**Option B:** This option is incorrect because there is no indication of a respiratory disease such as asthma or chronic obstructive pulmonary disease (COPD) in the description. The symptoms of a panic attack are distinct from those of a respiratory disease, and the absence of physical examination abnormalities makes this diagnosis less likely.

**Option C:** This option is incorrect because there is no mention of a traumatic event or a previous history of post-traumatic stress disorder (PTSD). While PTSD can cause symptoms of anxiety and hypervigilance, the sudden onset of symptoms in the absence of a traumatic event makes this diagnosis less likely.

**Option D:** This option is incorrect because there is no indication of a systemic infection or inflammation in the description. The symptoms of a panic attack are distinct from those of a systemic infection or inflammation, and the absence of physical examination abnormalities makes this diagnosis less likely.

**Clinical Pearl / High-Yield Fact**
Panic attacks can be triggered by a variety of factors, including stress, anxiety, and certain medications. A thorough history and physical examination are essential in differentiating panic attacks from other medical conditions that may present with similar symptoms.

**Correct Answer: A. Panic disorder**