**Core Concept**
Cluster headaches are a type of primary headache disorder characterized by severe, unilateral orbital and temporal pain, often accompanied by autonomic symptoms such as conjunctival injection and lacrimation. They typically occur in clusters or cycles, with multiple headaches occurring at the same time of day and year.

**Why the Correct Answer is Right**
Bilateral photophobia is not a characteristic feature of cluster headaches. Cluster headaches are typically associated with unilateral photophobia, which means that light on the affected side of the face exacerbates the pain. This is due to the activation of the trigeminal nerve's ophthalmic division, which is responsible for transmitting pain signals from the eye to the brain. Bilateral photophobia, on the other hand, is more commonly seen in conditions such as migraines.

**Why Each Wrong Option is Incorrect**
**Option A:** Periodic pain is a characteristic feature of cluster headaches, as they often occur in clusters or cycles.
**Option B:** Conjunctival injection is a common autonomic symptom of cluster headaches, caused by the release of neuropeptides such as substance P and calcitonin gene-related peptide (CGRP) from the trigeminal nerve terminals.
**Option D:** Cluster headaches are often resistant to oral sumatriptan, which is why intranasal or subcutaneous administration is preferred.

**Clinical Pearl / High-Yield Fact**
Cluster headaches are often referred to as "suicide headaches" due to their intense, debilitating pain and the fact that some patients have attempted to end their lives during an episode.

✓ Correct Answer: C. Bilateral photophobia