Ans. is 'a' i.e., Creutzfeldt-Jakob disease o On microscopic examination, in Creutzfeldt-Jakob disease the pathognomonic finding is a spongiform transformation of the cerebral cortex and, often, deep gray matter structures (caudate, putamen); this consists of a multifocal process that results in the uneven formation of small, apparently empty, microscopic vacuoles of varying sizes within the neuropil and sometimes in the perikaryon of neurons. In advanced cases, there is severe neuronal loss, reactive gliosis, and sometimes expansion of the vacuolated areas into cystlike spaces ("status spongiosus").o Kuru plaques are extracellular deposits of aggregated abnormal protein; they are Congo red-positive as well as PAS-positive and occur in the cerebellum in cases of Gerstmann-Strdussler-Scheinker syndrome; they are present in abundance in the cerebral cortex in cases of variant CJD.o In all forms of prion disease, immunohistochemical staining demonstrates the presence of proteinase-K-resistant PrPsc in tissue.o Fatal familial insomnia does not show spongiform pathology. Instead, the most striking alteration is neuronal loss and reactive gliosis in the anterior ventral and dorsomedial nuclei of the thalamus; neuronal loss is also prominent in the inferior olivary nuclei.Some important questions on Prions diseaseo Disease confined to nervous system.o Caused by infection proteinaceous particles called prions.o Characterized by neurodegeneration and spongiform changes.o Usually have long incubation period.o Disease is usually chronic progressive disease.o The disease is invariably fatal.o The host shows no inflammatory response and no immune res pons.