Ricin is a toxin which destroys the ability of an organism to translate RNA and make critical proteins. Markov died because this happened when he was injected with ricin, and by four days later (when he died) enough critical proteins were non-functional and unreplaced that this led to his death.
Work Step by Step
Review earlier chapters on gene expression, especially translation. You will see that translation by ribosomes is critical since proteins wear out and are destroyed (see ubiquitinylation), meaning that they must be replaced. Ricin stops this.