**Core Concept**
Temporal arteritis, also known as giant cell arteritis, is a form of large vessel vasculitis that primarily affects the branches of the external carotid artery, including the temporal artery. This condition is characterized by chronic inflammation of these blood vessels, leading to their narrowing and eventual occlusion. This process can result in significant morbidity, including visual loss, if left untreated.

**Why the Correct Answer is Right**
The patient's symptoms of severe unilateral headache and blindness are classic presentations of temporal arteritis. The presence of a thick cord-like structure on the lateral side of the head, known as a temporal artery, is a key diagnostic finding. This structure is often tender to the touch and can be palpated as a cord-like mass. The elevated erythrocyte sedimentation rate (ESR) of 80 mm/hr in the first hour is also consistent with this diagnosis, as it indicates a significant inflammatory response.

**Why Each Wrong Option is Incorrect**
**Option A:** Migraine is a type of headache disorder that can present with unilateral pain, but it typically does not cause blindness and is not associated with a thick cord-like structure on the lateral side of the head.

**Option B:** Cluster headache is a severe form of headache that typically affects the same side of the head as the patient's eye, but it is not associated with a thick cord-like structure or blindness.

**Option D:** Sinusitis is an infection of the sinuses that can cause headaches, but it is not associated with a thick cord-like structure on the lateral side of the head or blindness.

**Clinical Pearl / High-Yield Fact**
In patients over the age of 50, a new headache that is severe, unilateral, and accompanied by visual disturbances or a tender temporal artery should raise suspicion for temporal arteritis. Prompt treatment with corticosteroids is essential to prevent visual loss and other complications.

**✓ Correct Answer: A. Temporal arteritis**