kā is optionally the substitute of ku when the word puruṣa follows.,
Thus कापुरुषः or कुपुरुषः ॥ This is an aprapta-vibhasha. In the sense of 'a little ईषद्', the substitution is compulsory; as ईषत् पुरुषः = का पुरुषः ॥,
विभाषा पुरुषे
Adhyāya 6 · Pāda 3 · Rule 106