A 10 month old child presents with two weeks history of fever, vomiting and alteration of sensorium Cranial CT scan reveals basal exudates and hydrocephalus. The most likely etiological agent is:
**Core Concept**
The question is testing the diagnosis of a specific infectious disease that affects the central nervous system (CNS) of infants, characterized by inflammation of the meninges and increased intracranial pressure.
**Why the Correct Answer is Right**
The clinical presentation of fever, vomiting, and alteration of sensorium in a 10-month-old child, along with basal exudates and hydrocephalus on cranial CT scan, is suggestive of **Bacterial Meningitis** caused by *Haemophilus influenzae type b* (Hib). This bacterium is a common cause of meningitis in infants and young children, and the presence of basal exudates and hydrocephalus is a classic finding in this condition. The inflammation and infection in the subarachnoid space lead to the production of cerebrospinal fluid (CSF) and increased intracranial pressure.
**Why Each Wrong Option is Incorrect**
* **Option A:** *Streptococcus pneumoniae* is a common cause of bacterial meningitis, but it is more commonly seen in adults and older children. The clinical presentation in this case is more suggestive of Hib.
* **Option B:** *Neisseria meningitidis* is another common cause of bacterial meningitis, typically seen in children and young adults. However, the presentation in this case is more consistent with Hib.
* **Option C:** *Viruses* such as enteroviruses and herpes simplex virus can cause meningitis, but the presence of basal exudates and hydrocephalus is more typical of bacterial meningitis.
**Clinical Pearl / High-Yield Fact**
Hib vaccine has been a crucial tool in reducing the incidence of bacterial meningitis in infants and young children. However, its use has led to a shift in the epidemiology of meningitis, with other pathogens such as S. pneumoniae and N. meningitidis becoming more common causes.
**Correct Answer:** D. **Haemophilus influenzae type b**