Parsing a XDocument

I have the following XDocument:

<ExtensionInfo><ObjectExtension /><AttributeExtension><Attribute Name="PV" ExtensionType="inputoutputextension" InheritedFromTagName="$CT_STQ_V2" /><Attribute Name="PV" ExtensionType="logdatachangeeventextension" InheritedFromTagName="$CT_STQ_V2" /><Attribute Name="STS" ExtensionType="inputoutputextension" InheritedFromTagName="$CT_STQ_V2" /><Attribute Name="STS" ExtensionType="booleanextension" InheritedFromTagName="$CT_STQ_V2" /><Attribute Name="STS" ExtensionType="alarmextension" InheritedFromTagName="$CT_STQ_V2" /><Attribute Name="STS" ExtensionType="logdatachangeeventextension" InheritedFromTagName="$CT_STQ_V2" /></AttributeExtension></ExtensionInfo>

I am trying to return $CT_STQ_V2 if and only if the Attribute Name="STS" and ExtensionType="alarmextension"

How can I use XDocument & LINQ query to get what I need. I need to parse through thousands of XML files so fast performance would be required. Any suggestions would help.

Thank you.