u is substituted for the vowel of car and phal before an affix beginning with t,
Thus चूर्तिः (= चरणं or ब्रह्मणः), प्रफुल्तिः and प्रफुल्ताः ॥ The anuvritti of यङ् लुकोरभ्यासस्य does not apply here, though present. See 8.2.55.,
ति च
Adhyāya 7 · Pāda 4 · Rule 89