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The situation described in the question pertains to the evaluation of a coagulation abnormality where the activated partial thromboplastin time (APTT) is prolonged. When APTT corrects after mixing the patient's plasma with normal plasma, it indicates that the deficiency is due to a factor present in the patient’s plasma that is also found in the normal plasma, allowing the coagulation process to return to normal levels.

In the case of a factor VIII deficiency, the mixing study demonstrates that normal plasma, which contains the missing factor, can restore the APTT to normal. This happens because factor VIII is essential for the activation of factor X, and its absence leads to prolonged APTT. In contrast, deficiencies in factors such as VII or X do not typically cause a correction of APTT with the addition of normal plasma since mixing with normal plasma would not compensate for the deficiency in factors that are extrinsic (VII) or downstream (X) and not directly involved in the intrinsic pathway that the APTT assesses.

Thus, the fact that APTT corrects after mixing strongly suggests a deficiency in factor VIII, indicating that this factor's presence is necessary for the proper function of the coagulation pathway assessed by APTT. Hence,