Final answer:
The 27-gauge hypodermic needle has the smaller lumen compared to the 18-gauge, 21-gauge, and 25-gauge needles because the higher gauge number indicates a thinner needle.
Step-by-step explanation:
When comparing the size of the lumen in hypodermic needles, the gauge number is inversely proportional to the diameter of the needle's lumen. This means that as the gauge number increases, the diameter (and thus the lumen) of the needle decreases. Therefore, among the options provided:
- 18-gauge
- 21-gauge
- 25-gauge
- 27-gauge
The 27-gauge needle has the smallest lumen. Needles with a higher gauge number are thinner and are used when a smaller volume of fluid is to be administered, or when the puncture site needs to be barely noticeable, such as in the case of cosmetic procedures or with patients who have thin veins.