。
She let out a scream at the flash of lightning.
Literal
She [topic-は] lightning [genitive-の] flash [object-を] saw-and scream [object-を] raised.
悲鳴をあげる is a fixed idiom for 'to let out a scream' — literally 'to raise a scream/cry,' the same あげる ('raise') used in 声をあげる ('raise one's voice'). 稲妻 ('lightning') is a poetic native word — the kanji 稲 ('rice plant') + 妻 ('wife/spouse') — historically pronounced いなづま (literally 'rice-plant wife'). The Japanese folk etymology held that lightning brought rice plants to maturity through some kind of 'marriage' to the rice fields, hence the name; it survives as a literary term, more vivid than the everyday 雷 ('thunder/lightning'). 閃光 ('flash') intensifies the visual effect — a sudden, blinding light.