**Question:** An adult hypertensive male presented with sudden onset severe headache and vomiting. On examination, there is marked neck rigidity and no focal neurological deficit was found. The symptoms are most likely due to:

A. Subarachnoid haemorrhage (SAH)
B. Encephalitis
C. Meningitis
D. Subdural haematoma

**Core Concept:**
The clinical scenario described in the question involves a patient presenting with severe headache, vomiting, and neck rigidity. These symptoms are indicative of an intracranial pathology, which can be classified into three main categories: Subarachnoid haemorrhage (SAH), Encephalitis, and Meningitis. Furthermore, the absence of focal neurological deficits helps to narrow down the possibilities.

**Why the Correct Answer is Right:**
In this case, the correct answer is **Subarachnoid haemorrhage (SAH)**. SAH is a potentially life-threatening complication of aneurysm rupture and is characterized by sudden onset severe headache, vomiting, and neck rigidity due to increased intracranial pressure. Encephalitis and Meningitis typically present with fever, lethargy, and focal neurological deficits, which was not observed in the described scenario.

**Why Each Wrong Option is Incorrect:**

A. **Encephalitis** usually presents with fever, lethargy, and focal neurological deficits. There is no mention of fever or focal neurological deficits in the given scenario.
B. **Encephalitis** is also characterized by diffuse brain inflammation, which is different from the focal neurological deficits observed in SAH.
C. **Meningitis** presents with fever, meningeal signs, and focal neurological deficits. The absence of fever and focal neurological deficits makes this option unlikely.
D. **Subdural haematoma** presents with a gradual onset of symptoms, and the presence of focal neurological deficits. The described symptoms are sudden onset, severe, and absence of focal neurological deficits makes this option less likely.

**Clinical Pearls:**

1. In clinical practice, history and examination findings are crucial in narrowing down the differential diagnosis.
2. The absence of fever and focal neurological deficits in the presented case makes SAH more likely among the given options.
3. Rapid assessment of patients with abrupt neurological symptoms and signs can help guide the diagnostic approach and initiate appropriate management as soon as possible to improve the patient's prognosis.

**Explanation of the Core Concept:**

Subarachnoid haemorrhage, also known as subarachnoid hemorrhage (SAH), is a life-threatening complication of ruptured cerebral aneurysm. The condition presents with sudden onset, severe headache, vomiting, and neck stiffness due to increased intracranial pressure. The Glasgow Coma Scale (GCS) is commonly used to assess a patient's neurological status and guide management decisions, including neurosurgical intervention if necessary.

**Why Each Wrong Option is Incorrect:**

A) Encephalitis is an infection of the brain, which presents with a gradual onset, fever, and focal neurological deficits. The patient's symptoms of sudden onset, severe headache, and absence