When I have an XDocument and I know a namespace but not its prefix, and I want to get a certain descendant with prefixes and not namespaces (e.g. exactly as it was in the original XML string), how can I?

For example, suppose I have

<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" xmlns:gdp="http://example.com/asdf">
   <soapenv:Header/>
   <soapenv:Body>
      <gdp:GetLotNum/>
   </soapenv:Body>
</soapenv:Envelope>

So perhaps I’d try:

XElement envelope = XElement.Parse(soapEnvelopeXml);
XNamespace envelopeNamespace = "http://schemas.xmlsoap.org/soap/envelope/";
return envelope.Descendants(envelopeNamespace + "Body").FirstOrDefault()?.ToString();

But that gives me full namespaces, resulting in:

<soapenv:Body xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/">
  <gdp:GetLotNum xmlns:gdp="http://example.com/asdf" />
</soapenv:Body>

Whereas I want the exact XML I started with:

<soapenv:Body>
  <gdp:GetLotNum/>
</soapenv:Body>

Can I just strip off the namespaces somehow? Or is there a more elegant way?