adding message to "alert(" + gvPk + ")

Hi, this code shows me an alter message box with the seleced PK, what about if I wan to add more message along with the PK #
I tried all different location like this but still not working  ... gvPK is  a string variable
alert(" + gvPk + ")
alert(" 'you choose' + gvPk + ")
alert(" "You choose" + gvPk + ")
gvPk +="you choose"
alert(" + gvPk + ")
protected void GridView4_SelectedIndexChanged(object sender, EventArgs e)
{
    string gvPk = GridView4.SelectedDataKey.Value.ToString();
    ClientScript.RegisterStartupScript(this.GetType(), "Alert", "alert(" + gvPk + ")", true);
}
...