A 6-month-old infant from rural Maharashtra presents to the immunisation clinic with a history of incomplete vaccination. The mother reports that the child received BCG and OPV-0 at birth, but missed the 6-week vaccines due to illness. On examination, the child is afebrile and clinically well. The health worker suspects possible adverse event following immunisation (AEFI) from a previous dose. Which investigation is most appropriate to confirm vaccine strain poliovirus in case of vaccine-derived poliovirus (VDPV) suspected paralysis?