The spelling of the word "Aphtheux" can be explained using the International Phonetic Alphabet (IPA). The first syllable "Aph" is pronounced as /æf/, with the "A" sound as in "cat" and "ph" sounding like "f". The second syllable "theux" is pronounced as /θu/, with "th" making a "θ" sound as in "think" and "eu" making a "u" sound as in "do". Therefore, the correct spelling of the word "Aphtheux" accurately represents its unique pronunciation.