I think that the correct answer is reactants of photosynthesis.
I think the answer should be medulla oblongata or foramen magnum.
The major nerves that leave brain should form a thick cord in the brain stem that called medulla oblongata. Medulla oblongata will pass a big hole in the base of the skull that called foramen magnum. Depends on what part the question asked(bone or tissue), it probably between those two options.
Answer:
Mushrooms indicate root or trunk problems
Mushrooms typically grow on dead wood or on some other matter that is rotting.