**Core Concept**
H. influenza meningitis is a bacterial infection of the meninges, the protective membranes surrounding the brain and spinal cord. To prevent relapse or complications, it is crucial to investigate for residual infection or complications before discharging the patient.

**Why the Correct Answer is Right**
The correct investigation is a **lumbar puncture (LP) or spinal tap** to check for cerebrospinal fluid (CSF) parameters, such as glucose, protein, and cell count. This helps to determine if the infection has resolved and if there are any signs of complications, like subarachnoid hemorrhage or hydrocephalus. The CSF analysis can also guide the administration of antibiotics and corticosteroids.

**Why Each Wrong Option is Incorrect**
**Option A:** **CT scan of the brain** is not a primary investigation before discharge in H. influenza meningitis, although it may be used to rule out complications like brain abscess or subdural empyema.

**Option B:** **Blood cultures** are essential in the initial diagnosis of H. influenza meningitis, but they are not a primary investigation before discharge.

**Option C:** **MRI of the brain and spine** is not a routine investigation before discharge in uncomplicated H. influenza meningitis.

**Clinical Pearl / High-Yield Fact**
A lumbar puncture is essential before discharge in bacterial meningitis to rule out complications like subarachnoid hemorrhage, which can present with a normal CSF glucose level.

**Correct Answer: C. MRI of the brain and spine**