患儿男,4岁。自幼青紫,生长发育落后,杵状指(趾),喜蹲踞,诊断为法洛四联症。20分钟前,在剧烈活动后突然发生昏厥,可能为( )。
A campus emergency ______ occur at any time of the day or night, weekend, or holiday, with little or no warning.
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