स्पृशोऽनुदके क्विन्

The affix क्विन् comes after the verb स्पृश् 'to touch' when it is in composition with a case-inflected word other than उदक 'water'.
By sutra 4 of this chapter, the anuvritti of the word सुपि is to be read into this sutra. It might be objected 'why do we not read the anuvritti of the word कर्म instead of सुपि, because the verb स्पृश् is a transitive verb, and after such verbs, the anuvritti of कर्म ought to come by sutra 4'? To this we say there is no harm in reading the anuvritti of सुपि in this case; because the word कर्तरि of the last sutra is also, in a way understood here, and indicates the collection of all sorts of agents whether objects, instruments &c., and this can only be when the first member of the compound is a word in general.

Thus घृतं स्पृशति = घृतस्पृश्, nom. sing. घृतस्पृक् 'he who touches clarified butter'. स्पृश् + क्विन् + सु = स्पृश् + वि + ० 6.1.68 = स्पृक् + ० 6.1.67 and 8.2.62. So also मन्त्रेण स्पृशति = मन्त्रस्पृक् 'who touches with hymns'; जलेन स्पृशति = जलस्पृक् 'who touches with water'. But उदकेन स्पृशति = उदकस्पर्शः; here we have the general affix अण्.

The न् of क्विन् is to distinguish this affix from affixes like क्विप् &c. For, had it been merely क्वि, then in sutra 8.2.62 we should have used क्वि प्रत्ययस्य कुः, and it would have been ambiguous, for क्वि might be explained as meaning both क्वि and क्विप्, as क्य means क्यष्, क्यप् &c. To remove this ambiguity, न् is added to make a distinctive affix. The न् cannot be for the sake of accent. 6.1.117, the verb being monosyllabic would of itself have taken that accent.