Final answer:
The most common gauge needle used for phlebotomy is the 21-gauge needle.
Step-by-step explanation:
The most common gauge needle used for phlebotomy is the 21-gauge needle. Phlebotomy is the process of drawing blood from a patient for diagnostic testing or blood donation.
The 21-gauge needle is chosen for its versatility and effectiveness in obtaining blood samples from veins of different sizes. Other commonly used needle gauges in phlebotomy include 18-gauge, 22-gauge, and 23-gauge needles.