How do I serialize an anonymous object for sending over a SOAP web service?

// Some code has been removed here to simplify the example.
[WebMethod(EnableSession = true)]
public Response GetPatientList() {
    var patientList = from patient in ((User)Session["user"]).Practice.Patients
                      select new {
                          patientID = patient.PatientID,
                          status = patient.Active ? "Active" : "Inactive",
                          patientIdentifier = patient.PatientIdentifier,
                          physician = (patient.Physician.FirstName + " " + patient.Physician.LastName).Trim(),
                          lastModified = patient.Visits.Max(v => v.VisitDate)
                      };
    return patientList;
}

public class Result {
    public bool success;
    public bool loggedIn;
    public string message;
}

public class PracticeInfoResult : Result {
    public string practiceName;
    public string address;
    public string city;
    public string state;
    public string zipCode;
    public string phone;
    public string fax;
}

[WebMethod(EnableSession = true)]
public PatientsResult GetPatientList(bool returnInactivePatients) {
    if (!IsLoggedIn()) {
        return new PatientsResult() {
            Success = false,
            LoggedIn = false,
            Message = "Not logged in"
        };
    }
    Func<IEnumerable<PatientResult>, IEnumerable<PatientResult>> filterActive =
        patientList => returnInactivePatients ? patientList : patientList.Where(p => p.Status == "Active");
    User u = (User)Session["user"];
    return new PatientsResult() {
        Success = true,
        LoggedIn = true,
        Message = "",
        Patients = filterActive((from p in u.Practice.Patients
                      select new PatientResult() {
                          PhysicianID = p.PhysicianID,
                          Status = p.Active ? "Active" : "Inactive",
                          PatientIdentifier = p.PatientIdentifier,
                          PatientID = p.PatientID,
                          LastVisit = p.Visits.Count > 0 ? p.Visits.Max(v => v.VisitDate).ToShortDateString() : "",
                          Physician = (p.Physician == null ? "" : p.Physician.FirstName + " " + p.Physician == null ? "" : p.Physician.LastName).Trim(),
                      })).ToList<PatientResult>()
    };
}
public class Result {
    public bool Success { get; set; }
    public bool LoggedIn { get; set; }
    public string Message { get; set; }
}
public class PatientsResult : Result {
    public List<PatientResult> Patients { get; set; }
}
public class PatientResult  {
    public int PatientID { get; set; }
    public string Status { get; set; }
    public string PatientIdentifier { get; set; }
    public string Physician { get; set; }
    public int? PhysicianID {get;set;}
    public string LastVisit { get; set; }
}