I feel your pain! I got exactly the same mark for the same subject. For me it was the last of all 7 ATPL exams and it was also the last exam session available prior to the Xmas break for ASL. I was soooo looking forward to Xmas holidays with a complete set of KDR's. Anyway like you I was certain I had gotten one question marked incorrectly wrong. I wrote to the newly appointed CASA person responsible for the exams asking him to check my question and my reasons why. To my surprise I did receive a reply and an explanation regarding the question I had enquired about. I had indeed answered the question incorrectly. The CASA person responsible was new to the job I believe and that may have been the reason why he went to the trouble of answering me. Good luck to you. I hope you have better luck than I did. Next time I sat that same exam (on the first available exam booking after Xmas) I passed with a much better result. I truly believe it's luck of the draw with the exam questions you get for systems.