AP has no fewer than 87 meanings in the list of Spanish medical abbreviations on the invaluable Cosnautas site, but as you suggest, the one that clearly fits this context is "anatomopatológico":

"39 anatomía patológica; anatomopatológico || ≡ anat. pat., anat. patol., APA || ◊ PATH (pathology)"
http://www.cosnautas.com/es/siglas/2659-ap

On the same site, Fernando Navarro comments in his Libro rojo (little short of a bible for Spanish medical translation) on the fact that where Spanish uses "anatomopatológico", English uses simply "pathological" (or "pathologic", which he prefers):

"1 [Histol.] En los países de habla inglesa se utiliza el término pathology para referirse a la anatomía patológica o morfopatología"
http://www.cosnautas.com/es/libro/37066-link?from_entry=1&ab...

"pathological examination (examen anatomopatológico), pathological findings (signos anatomopatológicos, datos anatomopatológicos o resultados anatomopatológicos, según el contexto), pathologic report (informe anatomopatológico), pathological specimen (muestra anatomopatológica o pieza anatomopatológica, según el contexto), pathological staging (estadificación anatomopatológica, determinación del estadio anatomopatológico), pathological studies (estudios anatomopatológicos)."

"The lesion was excised and pathologic examination confirmed the diagnosis of subungual exostosis. [...] Pathologic analysis of the lesion revealed “skin with bone and soft tissue, consistent with exostosis.”"
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3748887/

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Note added at 4 hrs (2017-10-01 19:27:55 GMT)
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So "estudio AP" will be pathologic(al) analysis (or study, but analysis is more usual in native EN texts), and "AP" alone will be "pathology".