The immediate cause of death is unknown.

造成死亡的直接原因不明。

《牛津词典》

Or there may be an as yet unknown cause of some cardiovascular pathology.

也可能有某种至今未知的原因导致心血管疾病。

youdao

The most common reason is a condition called idiopathic (unknown cause) thrombocytopenia (low platelets) purpura (ITP). Platelets are blood cells which aid is blood clotting.

最常见的原因是突发性(原因不明)低血小板紫癜(itp)，血小板是帮助血液凝结的血细胞(其实血小板不具有细胞结构——译者注)。

youdao

The cause is unknown -presumed to be multifactorial- with signs noted in early childhood related to abnormalities in behavior, language, and social development.

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The cause of pre-eclampsia is unknown, but it is clear that the placenta fails to invade into the wall of the mother's uterus.

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She has not aborted the seven-week-old fetus, but it is unknown whether the mistake will cause birth defects or other complications.

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