**Question:** A 64-year-old lady Kamla complains of severe unilateral headache on the right side and blindness for 2 days. On examination, there is a thick cord-like structure on the lateral side of the head. The ESR (erythrocyte sedimentation rate) is 80 mm/hr. in the first hour. The most likely diagnosis is:

A. Subarachnoid haemorrhage
B. Carotid artery dissection
C. Meningitis
D. Encephalitis

**Core Concept:** The key factors in diagnosing these conditions involve the clinical presentation, examination findings, and laboratory results. In this case, we have:

1. Severe unilateral headache and blindness: These symptoms are indicative of a cerebral event affecting the right side of the brain.
2. Cord-like structure on the lateral side of the head: This suggests a potential arterial source of the problem.
3. Erythrocyte sedimentation rate (ESR) is elevated (80 mm/hr in the first hour), which indicates inflammation or increased vascular permeability.

**Why the Correct Answer is Right:**

The correct answer is **Subarachnoid haemorrhage (SAH)**. The combination of symptoms, examination findings, and laboratory results align with this diagnosis.

- Severe unilateral headache and blindness point to a cerebral event affecting the right side of the brain, which is a characteristic presentation of SAH.
- The cord-like structure on the lateral side of the head suggests an arterial source, which is consistent with SAH.
- The elevated ESR indicates inflammation or increased vascular permeability, which is a common feature in SAH.

**Why Other Options are Incorrect:**

- Option **B (Carotid artery dissection**): This condition usually presents with a more diffuse headache, neck pain, and sometimes a palpable carotid artery. The cord-like structure and elevated ESR are not specific to carotid artery dissection.
- Option **C (Meningitis)**: Meningitis typically presents with fever, neck rigidity, and a non-cord-like structure on examination. Elevated ESR is not specific to meningitis.
- Option **D (Encephalitis)**: Encephalitis is associated with fever, focal neurological deficits, and a non-cord-like structure on examination. An elevated ESR is not specific to encephalitis.

**Clinical Pearls:**

1. SAH is a potentially life-threatening neurological emergency that requires prompt investigation and treatment.
2. The combination of symptoms, examination findings, and elevated ESR can lead to a strong suspicion of SAH, but definitive diagnosis requires further laboratory tests such as CT scan or angiography.
3. The cord-like structure on examination may be a clue to SAH, but it is not pathognomonic and can be seen in other conditions.