I figured this out. For unknown reasons, the C:\arduino-1.5.3-Intel.1.0.4\hardware\arduino\x86\tools\izmir\clupload_win.sh file was missing. Even reinstalling/repairing the Arduino for Windows application did not restore it. I needed to drill down into the zip file and manually extract it to the directory. After that, the Arduino IDE is functional again. I'll update this post if I figure out the sequence of events that led to the file being removed.
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