1public static SPListItem GetListItemFromLookup(SPListItem item, string lookupTableName, string key)
2{
3    SPFieldLookup field = item.Fields[key] as SPFieldLookup;
4    if (field != null && item[key] != null)
5    {
6        SPFieldLookupValue fieldValue = field.GetFieldValue(item[key].ToString()) as SPFieldLookupValue;
7        if (fieldValue != null)
8        {
9            return item.Web.Lists[lookupTableName].GetItemById(fieldValue.LookupId);
10        }
11    }
12    return null;
13}