I scrubbed into a surgery involving a lesion that looked pretty much like the one shown above. This appeared in a 40 year old female secretary after she applied lotion to her wrists and felt a pop. The lump was initially soft and painless, but over the last few months, it has grown in size, become harder, and caused a little discomfort. The mass transilluminates.
Challenge: What is the most likely diagnosis?
Image shown under GNU Free Documentation License.