Here is what you want:
<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet version="2.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:output method="text" version="1.0" encoding="UTF-8" indent="yes"/>
<xsl:variable name="newline">
<xsl:text> </xsl:text>
</xsl:variable>
<xsl:variable name="carriagereturn">
<xsl:text> </xsl:text>
</xsl:variable>
<xsl:template match="@*|node()">
<xsl:apply-templates select="@*|node()" />
</xsl:template>
<xsl:template match="/">
<xsl:text>ArticleID|CreatedDate|MeSH|IsMajor</xsl:text>
<xsl:value-of select="$carriagereturn" />
<xsl:apply-templates select="@*|node()" />
</xsl:template>
<xsl:template match="DescriptorName">
<xsl:value-of select="ancestor::MedlineCitation/PMID" />
<xsl:text>|</xsl:text>
<xsl:value-of select="ancestor::MedlineCitation/DateCreated/Year" />
<xsl:value-of select="ancestor::MedlineCitation/DateCreated/Month" />
<xsl:value-of select="ancestor::MedlineCitation/DateCreated/Day" />
<xsl:text>|</xsl:text>
<xsl:value-of select="." />
<xsl:text>|</xsl:text>
<xsl:value-of select="@MajorTopicYN" />
<xsl:value-of select="$carriagereturn" />
</xsl:template>
</xsl:stylesheet>
Good luck with it!