The word "hyporadii" is spelled with a "y" after "hypo" because it follows the rule that "y" is used as a vowel when it is not the first letter in a syllable. In this case, "y" represents the sounds [ɪ] and [aɪ], which can be heard in the pronunciation of "hi-puh-rey-dee-ahy" in IPA phonetic transcription. The word refers to the lower branches of an artery and is commonly used in medical terminology.