This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Systemic Bacteriology (Gram Positive Bacilli, Gram Negative Bacilli), Subject name: Microbiology
In salmonella infection, true statement is: 
Select and explain. 
A. Flagellar antigen forms chalky granular clumps, B. Somatic antigen is a protein, C. Absence of Vi antibody indicate poor prognosis, D. Serological classification is by Craige's scheme
C: Absence of Vi antibody indicate poor prognosis
Answer: C: Absence of Vi antibody indicate poor prognosis 
Explanation: Absence of Vi antibody indicate poor prognosis in salmonella infection is the true statement among the options given above. Option 1 Option 2 Somatic antigen (O Ag): Pa of cell wall lipopolysaccharide (LPS). Flagellar antigen (H Ag): Made-up of proteins: flagellin, confers motility. Option 3 The complete absence of the Vi antibody in a proven case of typhoid fever indicates poor prognosis Option 4 Phase variation: Conversion of isolates from one phase to another is achieved by using Craigie's tube method. Note: Vi Antibodies are not seen in normal population.