If a believer who is conscious of having committed a mortal sin eats and drinks the consecrated bread and wine, does he or she still receive the Body and Blood of Christ?

Yes. The attitude or disposition of the recipient cannot change what the consecrated bread and wine are. The question here is thus not primarily about the nature of the Real Presence, but about how sin affects the relationship between an individual and the Lord. Before one steps forward to receive the Body and Blood of Christ in Holy Communion, one needs to be in a right relationship with the Lord and his Mystical Body, the Church – that is, in a state of grace, free of all mortal sin. While sin damages, and can even destroy, that relationship, the sacrament of Penance can restore it. St. Paul tells us that “whoever eats the bread or drinks the cup of the Lord unworthily will have to answer for the body and blood of the Lord. A person should examine himself, and so eat the bread and drink the cup” (1 Cor 11:27-28). Anyone who is conscious of having committed a mortal sin should be reconciled through the sacrament of Penance before receiving the Body and Blood of Christ, unless a grave reason exists for doing so and there is no opportunity for confession. In this case, the person is to be mindful of the obligation to make an act of perfect contrition, that is, an act of sorrow for sins that “arises from a love by which God is loved above all else” ( Catechism, no. 1452). The act of perfect contrition must be accompanied by the firm intention of making a sacramental confession as soon as possible.