Complex sentence: Small specimens may resemble the common Portobello mushroom to non-experts, but just a piece of an "A. virosa" in a soup with hundreds of edible mushrooms is poisonous enough to kill all who eat the soup.
Split into simpler sentences: Small specimens may resemble the common Portobello mushroom to non-experts. But just a part of an "A. virosa" in a soup with hundereds of edible mushrooms may kill all who eat the soup.