Final answer:
Paresthesia of the perineum and buttocks is the sign of cauda equina compression among the options given, which is a symptom associated with saddle anesthesia, a classic indication of this condition.
Step-by-step explanation:
Cauda equina syndrome is a serious condition caused by compression of the nerves in the lower segment of the spinal canal and may present with a variety of symptoms. Given the choice of symptoms in the question, paresthesia of the perineum and buttocks is a sign of cauda equina compression. This sensation of tingling, numbness, or a 'pins and needles' feeling in the saddle area (around the buttocks, perineum, and inner surfaces of the thighs) is known as saddle anesthesia and is a classic symptom of this condition.
While other symptoms like gastrocnemius weakness, a reduced or absent ankle reflex, and numbness in the lateral foot can be associated with lower back pathologies, they are not as specific to cauda equina syndrome as saddle anesthesia. With a herniated disc at L5-S1, there can be pressure on the nerves that affect the areas related to the cauda equina. Therefore, in this context, the student should be able to identify paresthesia of the perineum and buttocks as a sign of cauda equina compression.