var q = from patient in ctx.dd_DDForm
    join mkab in ctx.hlt_MKAB on patient.MKABGuid equals mkab.UGUID
    group mkb by new {mkab.MKAID, patient.Name, patient.Family, patient.Patronymic} into x
    select new MKAB{
        BirthDay = x.Key.DR,
        Name = x.Key.Family,
        ....
    }