A patient presents with a headache, high fever and meningismus. Within 3 days he becomes unconscious. Most probable causative agent-
**Core Concept**
Bacterial meningitis is a severe infection of the meninges, the protective membranes surrounding the brain and spinal cord. It can cause inflammation, swelling, and potentially life-threatening complications if left untreated.
**Why the Correct Answer is Right**
The patient's symptoms, such as headache, high fever, and meningismus (stiff neck), are classic signs of bacterial meningitis. The rapid progression to unconsciousness within 3 days is also consistent with this diagnosis. The causative agents of bacterial meningitis are typically Gram-positive cocci, such as *Streptococcus pneumoniae*, or Gram-negative bacilli, such as *Neisseria meningitidis*. These pathogens can invade the meninges and trigger a severe inflammatory response.
**Why Each Wrong Option is Incorrect**
* **Option A:** While viral meningitis can present with similar symptoms, the rapid progression to unconsciousness within 3 days is less typical for this condition. Viral meningitis usually has a more gradual course.
* **Option B:** *Listeria monocytogenes* is a possible cause of bacterial meningitis, particularly in immunocompromised individuals or the elderly. However, it is less common than *Streptococcus pneumoniae* or *Neisseria meningitidis*.
* **Option C:** *Haemophilus influenzae* type b (Hib) was a common cause of bacterial meningitis in children before the introduction of the Hib vaccine. While it can still occur in unvaccinated individuals, it is less likely in this case.
**Clinical Pearl / High-Yield Fact**
In patients with suspected bacterial meningitis, prompt initiation of empiric antibiotic therapy is critical. The choice of antibiotics should cover the most common causative agents, such as *Streptococcus pneumoniae* and *Neisseria meningitidis*.
**Correct Answer: D. *Streptococcus pneumoniae***