The spelling of the word "most exertive" can be explained using IPA phonetic transcription. The first syllable "mo" is pronounced as /moʊ/, which has the diphthong "ow" sound. The second syllable "st" is pronounced as /st/, with the "s" sound followed by the consonant blend "t". Finally, the third syllable "ex-tive" is pronounced as /ɪkˈstɪv/, with the short "i" sound followed by the consonant blend "kst" and the long "i" sound. Together, these syllables form the word "most exertive", which means the most demanding or difficult.