**Core Concept**
When using an anaesthetic agent with a vasoconstrictor, it's essential to consider the potential risks and benefits, particularly in specific clinical scenarios where tissue oxygenation is crucial. The combination of an anaesthetic and a vasoconstrictor can lead to reduced blood flow and potentially worsen tissue hypoxia.

**Why the Correct Answer is Right**
In digital block procedures, such as ring block for finger or toe surgery, the anaesthetic agent with vasoconstrictor can cause vasoconstriction, leading to reduced blood flow to the digits. This can result in compromised tissue oxygenation and potentially severe complications, including necrosis or gangrene. The use of vasoconstrictors in digital blocks is generally contraindicated due to the high risk of these adverse outcomes.

**Why Each Wrong Option is Incorrect**
**Option B:** Spinal block involves injecting the anaesthetic into the subarachnoid space, which does not significantly impact peripheral blood flow. The use of vasoconstrictors in spinal block is not typically contraindicated.
**Option C:** Epidural block, similar to spinal block, involves injecting the anaesthetic into the epidural space, which does not directly affect peripheral blood flow. Vasoconstrictors may be used cautiously in epidural blocks, depending on the specific clinical scenario.
**Option D:** Regional anaesthesia is a broad term that encompasses various techniques, including digital blocks. However, the question specifically asks for a contraindication, and digital blocks are a more precise scenario where the use of vasoconstrictors is contraindicated.

**Clinical Pearl / High-Yield Fact**
When using anaesthetic agents with vasoconstrictors, always consider the specific clinical scenario and potential risks of tissue hypoxia. In digital blocks, it's essential to use anaesthetics without vasoconstrictors or alternatives that do not compromise blood flow.

**✓ Correct Answer: A. Digital block**