<prompt> Most serious form of Rickettsial disease -Scrub typhusQ-feverTrench feverRocky mountain spotted fever  <response>: Ans. is d i.e.,Rocky mountain spotted fever Among the Rickettsial diseases, Rocky mountain spotted fever is the most common and the most seriousThe microbial chalonge R rickettsii causative agent of RMSF is the most virulent Rickettsia -- Clinical microbiology 