The main subject of this excerpt is about how mad cow disease starts in the brain and can affect humans.
What caused the mad cow disease?
The disease was caused by contaminated cattle feed that contains the prion agent that affected the cows and were transferred to the cattle rearers and farmers.
The mad cow disease manifest in human as depression, loss of coordination, dementia etc.
Hence, the main subject of this excerpt is about how mad cow disease starts in the brain and can affect humans.
Therefore, the Option A is correct.
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