युवोरनाकौ

For यु and वु (nasalized) in an affix, are substituted, respectively, अन and अक।
Karika

युवोश्चेद् द्वित्वनिर्द्देशो द्वित्वे यण्तु प्रसज्यते ।
अथ चेदेकवद्भावः कथं पुंवद्भवेदयम् ॥
द्वित्ये नैगमिको लोप एकत्वे नुमनित्यता ।
आशिष्यत्वाद्धि लिङ्गस्य पुंस्त्वं वेहि समाश्रितम् ॥

यु and वु are taken here as stripped of all other indicatory letters, and the semi-vowels are also to be understood to have been nasalised. The अन replaces यु, and अक replaces वु ॥ Thus ल्यु 3.1.134 = अन, as नन्द + ल्यु = नन्दनः रमणः ॥ So also व्यु and व्युल् 4.3.23, as सायंतनः, चिरंतनः ॥ वु we find in ण्वुल् 3.1.133, as कृ+ण्वुल् = कारकः, हारकः ॥ So also वुन् 4.3.98 as वासुदेवकः, अर्जुनकः ॥

Why do we say nasalised यु and वु ? Observe ऊर्णाया युस् 5.2.123 Here the यु is not replaced by अन, and we have ऊर्णायुः ॥ So also in भुजिमृङ्भ्यां युक्त्युकौ ॥ Here the यु of युक् and त्युक् (Unadi III.21) are not replaced by अन; as भुज्युः and मृत्युः ॥ In the affixes above mentioned, the semi-vowel is not considered to have the nasal. There are no visible marks of nasality on any affixes, but the maxim is प्रतिज्ञानुनासिक्याः पाणिनीयाः ॥

The word युवोः is the Genitive Singular of युवु considered as a single word, i.e. a Samahara Dvandva compound in the singular, and such compounds are always neuter 2.4.17. The Genitive Singular of युवु is therefore युवुनः the augment being added by 7.1.73. The anomalous form युवोः shows the existence of the maxim that a rule ordaining an augment is not universally valid (अनित्यमागमशासनम्) ॥ Or the युवु may be considered as masculine Dvandva, and then it proves the maxim that the gender need not be taught, the usage of the people settles the gender of a word (लिङ्गमशिष्यं, लोकाश्रयत्वाल्लिङ्गस्य)॥ If, however, युवू be considered an itaretara-yoga Dvandva compound, then its Genitive dual will be युव्वोः, one व् is elided as a Chhandas irregularity, or the ऊ is elided, and we have the युव् + ओः = युवोः an anomalous dual.

Karika :- If युवू is taken as a dual, then there ought to be a semi-vowel (i.e. युव्वोः); if it is considered as Aggregate Singular, then it ought to be neuter, how is then युवोः masculine? (To this we answer), if it be taken as dual युवू, then there is elision of the यणादेश (i.e. व्); if it be taken as singular युवु, then the augment नुम् which comes in the Genitive Singular of Neuters, does not come here, proving that the rule of नुम् augment is Anitya (not of universal application). Or the word युवु is taken as masculine, because the Gender depends upon usage.