**Question:** An 18-year-old boy is brought to the hospital with loss of consciousness. He regains consciousness in the ER, and GCS on admission is 12/15. CNS examination is normal and auscultation reveals a narrow split S2 and clear lungs. ECG was done. Diagnosis is?

A. Cardiac arrest
B. Acute respiratory distress syndrome (ARDS)
C. Subarachnoid hemorrhage (SAH)
D. Encephalitis

**Core Concept:**

The clinical scenario presented involves a young patient with a history of loss of consciousness, normal neurological examination, clear lungs, and a narrow split second heart sound (S2) on auscultation, which suggests a cardiac cause.

**Why the Correct Answer is Right:**

The correct answer is D. Encephalitis, because the clinical presentation described does not fit the criteria for the other options:

1. A) Cardiac arrest usually presents with altered mental status, no history of loss of consciousness, and a normal neurological examination.
2. B) Acute respiratory distress syndrome (ARDS) is characterized by respiratory distress, hypoxia, and bilateral lung infiltrates on chest X-ray, not described in the given case.
3. C) Subarachnoid hemorrhage (SAH) is characterized by sudden severe headache, focal neurological deficits, and often Glasgow Coma Scale (GCS) scores lower than 12, not the score of 12/15.

**Why Each Wrong Option is Invalid:**

A) Cardiac arrest: Mentioned above – altered mental status, no history of loss of consciousness, and normal neurological examination.
B) Acute respiratory distress syndrome (ARDS): Described above, not matching the clinical presentation.
C) Subarachnoid hemorrhage (SAH): GCS score lower than 12 is usually seen in SAH patients, but the patient's GCS on admission is 12/15, making SAH an unlikely diagnosis.

**Clinical Pearl:**

A narrow second heart sound (split S2) is commonly seen in heart conditions like pulmonary hypertension or right heart failure, which may not be directly related to the neurological symptoms presented by the patient. Encephalitis is the most plausible diagnosis in this scenario, given the patient's history of loss of consciousness and normal neurological examination. Encephalitis is an inflammation of the brain and can cause fever, seizures, and altered mental status. The normal neurological examination is a clue to rule out other neurological causes like stroke or traumatic brain injury.

In conclusion, the narrow split S2 and normal neurological examination point towards cardiac etiologies like encephalitis, which is why option D is the correct answer.