**Question:** A man with very high fever shows strongly positive agglutination test with OXK antigen. The most likely diagnosis is-

A. Diphtheria
B. Typhoid fever
C. Pneumococcal pneumonia
D. Meningococcal meningitis

**Correct Answer:** D. Meningococcal meningitis

**Core Concept:**
Agglutination test is a serological test used to detect the presence of specific antibodies or antigens in a patient's blood sample. In this case, the strongly positive result with OXK antigen suggests that the patient has antibodies against a specific bacterial antigen.

**Why the Correct Answer is Right:**
The OXK antigen is specific to Neisseria meningitidis, the causative agent of meningococcal meningitis. Meningococcal meningitis is a life-threatening bacterial infection of the meninges, the protective membranes covering the brain and spinal cord. The high fever and positive agglutination test indicate severe infection, and the presence of OXK antigen confirms the diagnosis of meningococcal meningitis.

**Why Each Wrong Option is Incorrect:**
A. Diphtheria: Diphtheria is a bacterial infection caused by Corynebacterium diphtheriae, and the agglutination test specific to diphtheria is OX19, not OXK.
B. Typhoid fever: Typhoid fever is caused by Salmonella typhi, and the agglutination test specific to typhoid is Widal test, not OXK.
C. Pneumococcal pneumonia: Pneumococcal pneumonia is caused by Streptococcus pneumoniae, and the agglutination test specific to pneumococcal pneumonia would be a serological test for polysaccharide antigens, not OXK.

**Clinical Pearl:** Meningococcal meningitis is a severe form of bacterial meningitis that requires immediate treatment, as it can rapidly progress to sepsis and multiple organ failure if not treated promptly. Prompt recognition and diagnosis are crucial for patient management and survival.