A young male is admitted with history of altered sensorium and hydrophobia. A clinical diagnosis of rabies was made and corneal scrapings were taken. Which is the best test to confirm his diagnosis?
**Core Concept**
The clinical presentation of altered sensorium and hydrophobia in a patient with a history of animal bite is suggestive of rabies, a viral encephalitis caused by the rabies virus. The virus affects the central nervous system, leading to characteristic neurologic symptoms.
**Why the Correct Answer is Right**
The best test to confirm the diagnosis of rabies is the direct fluorescent antibody (DFA) test on corneal scrapings. This test detects the presence of rabies virus antigen in the corneal tissue using fluorescent-labeled antibodies. The DFA test is highly sensitive and specific, allowing for rapid confirmation of the diagnosis. The test typically takes 30 minutes to 1 hour to perform, making it a valuable tool for early diagnosis and post-exposure prophylaxis (PEP).
**Why Each Wrong Option is Incorrect**
**Option A:** PCR (Polymerase Chain Reaction) on saliva or corneal scrapings is not the best initial test for confirming rabies diagnosis, although it can be used as a supplementary test to confirm the presence of the virus.
**Option B:** Histopathological examination of the brain tissue is the gold standard for diagnosing rabies, but it is an invasive procedure that requires a biopsy, which is not feasible in a living patient.
**Option C:** ELISA (Enzyme-Linked Immunosorbent Assay) on serum is not a reliable test for diagnosing rabies, as the antibodies may not be detectable until late in the disease course.
**Option D:** Culture of the virus from corneal scrapings is not a practical or reliable method for diagnosing rabies, as the virus is difficult to culture and may not be present in sufficient quantities.
**Clinical Pearl / High-Yield Fact**
In cases of suspected rabies, it is essential to obtain corneal scrapings or saliva samples from the patient as soon as possible to facilitate rapid diagnosis and initiation of post-exposure prophylaxis (PEP).
**Correct Answer:** C.