Sayyed Mohammad Al-Musawi, Sayyed Mohammad al-Musawi is originally from Iraq and heads up the World Ahlul Bayt Islamic League in London. Other than being involved in various humanitarian projects, he frequently responds to... Answered 1 year ago

If the pen mark is just a color from ink which has no layer on your skin, your prayer is valid. If the pen mark is thick enough and formed a layer preventing water from reaching to the skin, then Wudhu was invalid if it was done after the layer is on the skin. If you do not know whether the layer was before or after performing the Wudhu, your Wudhu is valid and prayer is valid as well.

Wassalam.