Enteric fever on fourth day is best diagnosed by:

Ans. (d) Blood cultureRef.:Harrison 18th edition, Chapter 153* The definitive diagnosis of enteric fever requires the isolation of S. typhi or S. paratyphi from blood, bone marrow, other sterile sites, rose spots, stool, or intestinal secretions.For diagnosis in the first week* Sensitivity of blood culture is only 40-80%, probably because of high rates of antibiotic use in endemic areas and the small quantities of S. typhi (i.e., <15 organisms/ mL).* Bone marrow culture is 55-90% sensitive, and, unlike that of blood culture, its yield is not reduced by up to 5 days of prior antibiotic therapy.* Culture of intestinal secretions (best obtained by a noninvasive duodenal string test) can be positive despite a negative bone marrow culture.