The first member of a tat-puruṣa samāsa preserves its original accent when the second member is the word pada denoting 'a pretext'.,
The word अपदेश means 'a pretext', 'a contrivance'. Thus मूत्र꣡पदेन प्रस्थितः, उच्चार꣡पदेन प्रस्थितः ॥ Gone pretext of voiding urine or excreta.
The word मूत्र is derived by adding the affix ष्ट्रन् to the root मुच्, the ऊ being substituted for उच् of मुच्, (Unadi. IV. 163), and has acute on the first 6.1.197 or it may be a word formed by घञ् to the root मूत्रयति ॥ The word उच्चार is also formed by घञ् and by 6.2.144 has acute on the final. The compounding takes place by 2.1.72 or it is an attributive compound.
Why do we say \when meaning a pretext?\ Observe विष्णोः पदम् = विष्णुपद꣡म् ॥,