**Core Concept**
Kernig's sign is a clinical indicator used to assess the presence of meningitis, specifically pyogenic meningitis. It is characterized by the inability to extend the knee from a flexed position beyond 90 degrees due to irritation of the meninges.

**Why the Correct Answer is Right**
Pyogenic meningitis is caused by bacterial infection, leading to inflammation of the meninges. Kernig's sign is a result of this inflammation, which causes stiffness and resistance to movement in the neck and back. The sign is elicited by flexing the patient's hip to 90 degrees and then attempting to extend the knee. If the patient experiences pain or resistance in the lower back or neck, Kernig's sign is positive, indicating pyogenic meningitis. The inflammation of the meninges in pyogenic meningitis irritates the nerve roots, leading to the characteristic symptom.

**Why Each Wrong Option is Incorrect**
**Option A:** Viral encephalitis is a different condition that affects the brain, not the meninges. While it can cause inflammation and irritation, it is not typically associated with Kernig's sign.
**Option C:** Brain abscess is a localized collection of pus in the brain, which may cause various symptoms, but Kernig's sign is not a characteristic finding.
**Option D:** All of the above is incorrect because Kernig's sign is specific to pyogenic meningitis and not a general indicator of meningitis or other neurological conditions.

**Clinical Pearl / High-Yield Fact**
Kernig's sign is a valuable clinical indicator that can help differentiate pyogenic meningitis from other neurological conditions. It is essential to remember that a positive Kernig's sign, combined with other clinical findings, can support the diagnosis of pyogenic meningitis.

**✓ Correct Answer: B. Pyogenic meningitis**