Final Answer:
Erasmo used coca leaves to diagnose his mother's illness by observing the change in color of the chewed leaves. He noticed that the coca leaves turned black, indicating the presence of poison in his mother's body. This led him to discover that her illness was caused by arsenic poisoning.
Step-by-step explanation:
Erasmo was a character in Gabriel García Márquez's novel "One Hundred Years of Solitude." In the story, Erasmo suspected that his mother, Rebeca, was being poisoned but couldn't identify the cause. He used coca leaves, which in the novel were believed to have diagnostic properties, to detect poison.
By chewing the coca leaves and observing their color change, Erasmo noted that the leaves turned black, indicating the presence of poison. This discovery eventually led to the revelation that his mother's illness was due to arsenic poisoning.