As those who follow my blog know, I have an interest in Mariology, and have even written a book on the topic:
In the book, I dedicate two chapters addressing the Immaculate Conception, one focusing on the purported biblical evidence and the second on the patristic testimony against such a teaching.
One of the more esoteric debates within Catholic Mariology is that of the debitum remotum, this being the doctrine that, although Mary was never actually under the influence of causes sufficient to produce original sin, nevertheless she ought to have been, being a daughter of Adam, and the debitum proximum, the doctrine that Mary was, in her own physical person, under the influence of a causal factor sufficient of itself to produce original sin in her soul (prescinding from the intervention of God).
Joseph Pohle (1852-1922), a Catholic dogmatic theologian who produced many learned tomes on Catholic theology, wrote the following addressing this debate in his study of Mariology:
The fact that Mary was preserved from original sin does not necessarily imply that she was exempt from the universal necessity or need of being subject to it (debitum peccati originalis).
Theologians generally hold that, though she was de facto exempt from original sin, Mary incurred the debitum contrahendi, because else her Immaculate Conception would not be an effect of the atonement.
We may distinguish a twofold debitum, proximate and remote. Debitum remotum merely signifies membership in the human race, based on the ordinary mode of propagation, i.e., sexual generation. Debitum proximum involves inclusion in the wilful act by which Adam, as the representative of the whole race, rejected the grace of God and implicated human nature in sin. The dogma of the Immaculate Conception is sufficiently safeguarded by admitting that Mary was subject to the debitum remotum. The view of some older Scotist theologians, that she had not even so much as a demitum remotum incurrendi paccatum originale, cannot be reconciled with the solemn formula by which Pope Pius defined the dogma of the Immaculate Conception.
Is it necessary to admit that there was also a debitum proximum? The majority of Catholic divines, following Suarez, contend that it is. The assumption of such a debitum, involving as it does the exemption of one sole individual from a strictly binding universal law, constitutes the Immaculate Conception a miracle and a far higher grace than it would be in the opposite hypothesis; but it does not sufficiently safeguard the soul of our Lordy against the possibility of contamination. (Joseph Pohle, Mariology: A Dogmatic Treatise on the Blessed Virgin Mary, Mother of God, with an Appendix on the Worship of the Saints, Relics, and Images, 39-40)