I have a black spot on my thumb that is

Patient

Q: I have a black spot on my thumb that is very slightly partially under the bottom of my thumbnail, and this extends past the cuticle to under the skin below the nail itself (not under). I do not recall injuring the area and it does not hurt. It has appeared suddenly and has been unchanged for the past week. Would appreciate any idea to what this is! Thanks!

Images: 
I have a black spot on my t...-1
Doctor

A:   Hello. Welcome to Ask The Doctor.
Thanks for trusting us for your health issues.
I have read your query in details and have studied the attached photograph too.

There are following main possibilities for such discoloration of nail:

1) Subungual Hematoma - In your case, this is the most likely possibility.
However, you don't give any history of injury to the nail but at times, sudden spontaneous trauma is so mild that It goes unnoticed but is sufficient enough to produce such hematoma under the skin.
It can resolve spontaneously in few weeks. If doesn't respond of its own, may be drained by an expert dermatologist.

2) Fungal infection: This is characterized by discoloration of the nail plate with or without swelling of nail folds. It is best managed by oral antifungals like Terbinafine/Itraconazole.

3) Melanoma - It is least likely possibility. Still I have mentioned in the differential diagnosis. However in this condition, the discoloration remains at a place and persists.

I will advise you to see your doctor so that he/she can examine you.
If required, he/she may order nail matrix biopsy for you which will reveal the confirmatory diagnosis.
You can share my opinion with him.
I hope it helps.
Stay Healthy.

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