A monosyllabic bha aṅga (stem) retains its original form without undergoing any change before the affixes 1. iṣṭha 2. iman and 3. īyas ,
Thus स्रजिष्ठः, स्रजीयान्, स्रजयति from स्रग्विन्, the विन् being elided by 5.3.65, the stem thus becoming monosyllabic. स्रजयति is the causative, formed on the model of इष्ठ by the Vartika under 6.4.155. So also स्रुचिष्टः, स्रुचीयान् and स्रुचयति from स्रुग्वत्, the मतुप् being elided as before. Why do we say 'a mono-syllabic stem? Observe वसिष्ठः from वसुमत् ॥ This rule is an exception to 6.4.155.
Vart:- राजन्य, मनुष्य and युवन् retain their original form unchanged, before the affix अक ॥ As, राजन्यानां समूहः = राजन्यकम्, मानुष्यकम् ॥ This debars the elision of य required by 6.4.15. So यूनो भावः = यौवनिकः with वुञ् affix 5.1.133, in supersession of 6.4.144.,