I think "troncal" also implies that a course is part of the CORE programme. "Asignatura anual" could be literally translated as a "one-year course". If you're sure it is optional, then "optional course" would also be an option!

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Note added at 7 mins (2004-12-14 15:13:44 GMT)
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anual.
I think it\'s the second meaning of \"anual\" (see below) that\'s being used here. Therefore, I\'d say \"one-year\" is more suitable than \"annual\" in English.

(Del lat. annuālis).
1. adj. Que sucede o se repite cada año.
2. adj. Que dura un año.



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Note added at 9 mins (2004-12-14 15:15:35 GMT)
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See below: three terms = one asignatura anual

[PDF] Arquitectura Técnica
File Format: PDF/Adobe Acrobat
... gráficos que se proponen para su realización a lo largo del curso, repartidos en
los tres trimestres virtuales en que puede dividirse esta asignatura anual. ...
www.ucam.edu/titulaciones/guia/arquitectura.pdf

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Note added at 11 mins (2004-12-14 15:18:04 GMT)
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http://www.ugr.es/~riletras/ext/guide.htm:

I think I\'d forget the \"optional idea\" unless you have other evidence in the text.

Asignatura anual [year-long subject]: This lasts from September to June. Exams take place in February and June/July. If you are coming for the whole academic year you can choose this subject, but if you are only coming from February to June, you will have to take into account that if you choose it, you will not have attended even half the course of this subject and therefore it will be more difficult to pass the final exams.

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Note added at 13 mins (2004-12-14 15:19:58 GMT)
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\"Year-long course\" would be another alternative.