This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Infection, Subject name: Medicine
A four-fold increase in the titre obtained in Weil- Felix reaction is diagnostic of - 
Select and explain. 
A. Rickettsial infection, B. Fungal infection, C. Spirochetal infection, D. Viral infection
A: Rickettsial infection
Answer: A: Rickettsial infection 
Explanation: In weil-felix test, host antibodies to various rickettsial species cause agglutination of Proteus bacteria because they cross-react with bacterial cell surface antigens. Reference: Harrison20th edition pg 1011