**Core Concept**
Adrenaline (epinephrine) is added to local anesthetics like lidocaine to prolong the duration of action by reducing systemic absorption and enhancing vasoconstriction at the site of injection. The concentration of adrenaline is expressed in a 1:X ratio, indicating 1 mg of adrenaline per X mg of solution.

**Why the Correct Answer is Right**
In clinical practice, lidocaine is commonly mixed with adrenaline at a concentration of **1:200,000**, meaning 1 mg of adrenaline per 200,000 mg of solution (or 1 mg/200 mL). This ratio provides sufficient vasoconstriction to minimize systemic absorption while minimizing the risk of toxicity. The 1:200,000 dilution is standard for most local anesthetic preparations and is widely accepted in anaesthesia guidelines.

**Why Each Wrong Option is Incorrect**
Option A: 0.180555556 – This is a decimal approximation of a fraction with no clinical meaning; it does not represent a standard adrenaline concentration ratio.
Option B: 1.430555556 – This value is numerically inconsistent with standard adrenaline ratios and lacks clinical relevance.
Option C: 1:20,000 – This is a higher concentration of adrenaline (1 mg per 20,000 mg), which significantly increases the risk of systemic toxicity and is not recommended for lidocaine mixtures.

**Clinical Pearl / High-Yield Fact**
Always use **1:200,000** adrenaline in lidocaine for most regional blocks. Avoid higher ratios (like 1:20,000) as they increase the risk of cardiac arrhythmias and hypertension.

✓ Correct Answer: D. 1:200000