Explanation:

The word 'अभ्यंतर' is formed by the combination of 'अभि' (Abhi) and 'अंतर' (Antar). In this combination, the 'इ' (i) sound at the end of 'अभि' changes to 'य' (ya) when followed by a vowel ('अ' in 'अंतर'). This transformation of 'इ' to 'य' (or 'उ' to 'व', 'ऋ' to 'र') is a defining characteristic of यण संधि (Yan Sandhi).