One option is to check for descendant's text:
from lxml import etree as ET
data = """<organizations>
<organization>
<orgID>152</orgID>
<orgName>This is A</orgName>
</organization>
<organization>
<orgID>1352</orgID>
<orgName>This is B</orgName>
</organization>
<organization>
<orgID>1522</orgID>
<orgName>This is C</orgName>
</organization>
<organization>
<orgID>1512</orgID>
<orgName>This is D</orgName>
</organization>
</organizations>"""
tree = ET.fromstring(data)
print tree.xpath('//organization[descendant::text()="1512"]/orgName/text()')
prints:
['This is D']