**Core Concept**
The diagnosis of pyogenic meningitis relies on the detection of bacterial pathogens and characteristic inflammatory changes in cerebrospinal fluid (CSF), primarily through CSF examination, which includes cell count, protein, glucose, and Gram stain.

**Why the Correct Answer is Right**
CSF examination is the cornerstone in diagnosing pyogenic meningitis. It reveals a marked increase in white blood cells (neutrophils), elevated protein, and reduced glucose levels. Gram staining of CSF can identify bacteria directly, while culture confirms the organism. This is a rapid, accessible, and essential diagnostic tool in clinical practice.

**Why Each Wrong Option is Incorrect**
Option A: Widal test detects *Salmonella* typhi and is used for typhoid fever, not meningitis. It is not specific or sensitive for bacterial meningitis.
Option B: CSF PCR is highly sensitive and can detect bacterial DNA, but it is not the first-line or primary diagnostic test. It is used as a supplementary tool, especially in cases with negative cultures.
Option D: PET scan is used in evaluating brain tumors or metabolic disorders, not in diagnosing bacterial meningitis. It has no role in acute meningitis diagnosis.

**Clinical Pearl / High-Yield Fact**
In pyogenic meningitis, CSF shows **high white blood cell count, high protein, and low glucose** — a classic triad. Always perform CSF examination in suspected meningitis cases; it is the most immediate and reliable diagnostic step.

✓ Correct Answer: C. CSF examination