A 15-year-old male is rushed to the emergency room with a temperature of 103degF, severe headache, and stiff neck. Upon physical examination, a petechial rash is observed all over his body. Suspecting meningitis, the physician orders a lumbar puncture, revealing gram-negative diplococci (Neisseria meningitidis) on Gram stain. The physician wishes to use a more sensitive test to confirm this as the causative agent. Which of the following tests combines features of gel diffusion and immune-electrophoresis and is applicable only to negatively charged antigens?

**Core Concept**
The question is testing the knowledge of a specific laboratory test used for confirming the causative agent of meningitis. The correct test combines features of gel diffusion and immune-electrophoresis and is applicable only to negatively charged antigens. This test is used to detect specific antibodies in the serum of a patient.

**Why the Correct Answer is Right**
The correct answer is the **Latex Agglutination test**, also known as the **Latex Slide Test** or **Slide Agglutination**. This test combines the principles of gel diffusion and immune-electrophoresis, where the negatively charged antigen (in this case, Neisseria meningitidis) reacts with specific antibodies in the serum, causing visible aggregation or agglutination of latex particles. This reaction occurs due to the electrostatic attraction between the negatively charged antigen and the positively charged latex particles, which are covalently linked to specific antibodies. The latex agglutination test is a rapid and sensitive method for detecting specific antibodies in the serum of a patient.

**Why Each Wrong Option is Incorrect**
* **Option A**: The **Counterimmunoelectrophoresis (CIEP)** test is a type of immunoelectrophoresis that involves the migration of both the antigen and antibody towards each other in a gel matrix. However, it is not a combination of gel diffusion and immune-electrophoresis, and it can detect both positively and negatively charged antigens.
* **Option B**: The **Enzyme-Linked Immunosorbent Assay (ELISA)** is a highly sensitive and specific test for detecting specific antibodies in the serum of a patient. However, it is not a combination of gel diffusion and immune-electrophoresis, and it does not involve the use of latex particles.

**Clinical Pearl / High-Yield Fact**
The latex agglutination test is a rapid and sensitive method for detecting specific antibodies in the serum of a patient, and it is often used in conjunction with Gram stain and culture to confirm the diagnosis of meningitis.

**Correct Answer:** **C**.

✓ Correct Answer: B. Counter immuno-electrophoresis (CIE)