1. Conformational isomers can differ around multiple sigma bonds. (objective scientific fact)

2. The sentence "conformational isomers differ by rotation around a sigma bond" grammatically excludes conformational isomers that differ around more than one sigma bond. (objective grammatical fact)

3. Considering point 1 and point 2, it follows that the current sentence structure of the card asserts an incorrect definition of conformational isomers, where rotations around multiple sigma bonds are excluded. (logically valid/sound and evidence-supported conclusion)

4. Possible corrections could include either "rotation around sigma bonds" (the most commonly used definitional verbiage), or more unambiguously, "rotation around one or more sigma bonds"

https://www.sciencedirect.com/topics/chemistry/conformational-isomer#:~:text=These%20different%20arrangements%20are%20obtained%20by%20rapid%20rotations%20around%20single%20bonds%2C