This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Anaesthesia
A patient was admitted epidural anaesthesia with 15 ml of 1.5%. Lignocaine with adrenaline for hernia surgery. He devoled hypotension and respiratory depression within 3 minutes after administration of block. The most common cause would be – 
Select and explain. 
A. Allergy to drug administered, B. Systemic toxicity to drug administered, C. Patient got vasovagal shock, D. Drug has entered the subarachnoid space
D: Drug has entered the subarachnoid space
Answer: D: Drug has entered the subarachnoid space 
Explanation: Hypotension and respiratory depression after epidural anesthesia suggest high spinal anesthesia.
High spinal anesthesia, i.e. spinal anesthesia ascending into the cervical levels, can occur following attempted epidural / caudal anesthesia if there is inadverent intrathecal injection. High spinal anesthesia causes severe hypotension, bradycardia, and respiratory insufficiency.

Causes of high spinal anesthesia :

Administration of excessive dose during spinal anesthesia,
Unusual sensitivity or spread of local anesthetic,
Failure to reduce standard dose in selected patients (elderly, pregnant, obese or very short),
Inadverent intrathecal injection during epidural anesthesia.