Final answer:
In medical terminology, a combining vowel is not used when the second word or root starts with a vowel, ensuring smooth pronunciation.
Step-by-step explanation:
A combining vowel is never used when the second word starts with a vowel. In medical terminology, a combining vowel (often "o" or "i") is not necessary when the word part it connects to begins with a vowel, as the two vowel sounds would naturally flow without the aid of an additional vowel. Therefore, in the context of medical terminology, the combining vowel’s purpose is to make word parts easier to pronounce when they come before a consonant or to join two root words smoothly.