XDocument: get the first child element of type 'a' or 'b'

I have the below markup, in which each item element must contain exactly one a, b, c, or d element, and in addition can contain a variety of additional elements. The position of the required (a, b... type) elements is not certain.

<root>
  <item>
    <a>...</a>
    ...
  <item>

  <item>
    ...
    <d>...</d>
  <item>

  <item>
    <c>...</c>
  <item>

  ...
</root>

Using XDocument, how can I express the following:

Return the first child element of type a, b, c, or d.

I'm currently using a chain of if-else statements, but this blasphemy must go; it seems way too much for such a seemingly easy task:

foreach (XElement xItem in xmlDoc.Root.Elements("item"))
{
    if (xItem.Element("a") != null)
    {
        // element <a>...</a> found
    }
    else if (xItem.Element("b") != null)
    {
        // element <b>...</b> found
    }
    ...
}