"I am lying on my back."
Translation:Я лежу на спине.
I understand that in Russian, possessive pronouns are rarely ever used with body parts, it is automatically assumed that you are talking about your own back here. If that weren't the case, you'd have to specify that it was someone else's. The same way you would never hear "У меня болит своя голова" normally for "my head hurts"
When спина was introduced earlier in this lesson, the first definition given on hover was "spine". I used "spine" in my answer and was marked wrong. Now I have found the word in my dictionary, and "spine" is not given as a definition - only "back". This dictionary gives a very different word for "spine". Why did DL give us "spine" as a definition if dictionaries do not support it and DL itself does not accept it as an answer?