Mycoplasma pneumoniae causes atypical pneumonia. The Weil-Felix test uses which cross-reactive antigen and is positive in which conditions, but NOT in Mycoplasma infection?

• A The Weil-Felix test uses Proteus vulgaris OX strains (OX19, OX2, OXK) as antigens; it is positive in rickettsial infections including typhus and spotted fevers, but NOT in mycoplasma pneumonia ✓
• B The Weil-Felix test uses Rickettsia-specific antigens purified from yolk sac cultures; it is positive in mycoplasma and Q fever
• C The Weil-Felix test uses Legionella pneumophila antigens and detects urinary antigen in Mycoplasma and Legionella co-infection
• D The Weil-Felix test uses Brucella melitensis antigens cross-reacting with rickettsial and mycoplasmal polysaccharide structures

Explanation

The Weil-Felix agglutination test exploits the cross-reactive polysaccharide antigens shared between Rickettsia species and certain Proteus vulgaris strains. The three relevant strains are OX-19 (agglutinated by anti-R. prowazekii/typhi antibodies in typhus group), OX-2 (agglutinated in spotted fever group with lower titre), and OXK (agglutinated in scrub typhus by R. tsutsugamushi antibodies). Importantly, Weil-Felix is NOT positive in Q fever (Coxiella burnetii), Rickettsia rickettsii infections weakly, or Mycoplasma pneumonia — Mycoplasma causes cold agglutinins (anti-I IgM) not Weil-Felix reactivity. The Weil-Felix test is non-specific and largely supplanted by IFA and ELISA.

Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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