I can see that the subject is actually Anesthesiology, not Microbiology. Here's the explanation:

**Core Concept**
Lignocaine, also known as lidocaine, is a local anesthetic commonly used for spinal anesthesia to block pain signals from the lower part of the body. The concentration of lignocaine used for spinal anesthesia is crucial to achieve effective analgesia with minimal side effects.

**Why the Correct Answer is Right**
A 5% solution of lignocaine is typically used for spinal anesthesia because it provides adequate analgesia for surgical procedures in the lower abdomen, perineum, and lower extremities. The higher concentration of lignocaine is necessary to ensure that the anesthetic effect is sufficient to cover the dermatomes involved in the surgical procedure. The addition of a vasoconstrictor, such as adrenaline, can help to prolong the duration of action and reduce the systemic toxicity of lignocaine.

**Why Each Wrong Option is Incorrect**
**Option A:** A 0.5% solution of lignocaine would be too dilute for spinal anesthesia, providing inadequate analgesia for surgical procedures.
**Option B:** A 1% solution of lignocaine is sometimes used for epidural anesthesia, but it is not the preferred concentration for spinal anesthesia.
**Option C:** A 2% solution of lignocaine may be used for certain procedures, such as ophthalmic or dental surgery, but it is not the standard concentration for spinal anesthesia.

**Clinical Pearl / High-Yield Fact**
For spinal anesthesia, lignocaine is usually combined with a vasoconstrictor, such as adrenaline, to prolong the duration of action and reduce the risk of systemic toxicity.

**Correct Answer:**
✓ Correct Answer: D. 5% solution