Step 1: Recall what GBS is. Guillain Barre Syndrome is an acute, immune mediated, demyelinating polyradiculoneuropathy, often following an infection. It attacks peripheral nerves and roots.
Step 2: Verify the true statements. Ascending paralysis is true: weakness classically starts in the legs and climbs upward. Flaccid paralysis is true: because it is a lower motor neuron, peripheral nerve process, the paralysis is flaccid with loss of reflexes (areflexia). Albumino-cytological dissociation is true: the CSF shows raised protein with a normal cell count, a hallmark finding.
Step 3: Find the false statement. A sensory level is a feature of a SPINAL CORD lesion (myelopathy), where there is a clear horizontal level below which sensation is lost. GBS is a peripheral nerve disease and does NOT produce a sensory level. So the statement "sensory level" is false for GBS.
Step 4: Confirm the exception. The question asks for the statement that is NOT true. That is the sensory level, option C.
Conclusion: A sensory level is not a feature of GBS, so the exception is option C.