Select the topic sentence.

1What is amnesia? 2Most of us think of a dramatic, total loss of memory brought on by a severe blow to the head. 3Amnesia is, however, a very subtle, and sometimes sinister, illness. 4While amnesia can result from a head injury, overwork and worry can also lead to memory lapses. 5In some cases, diagnosis is difficult because these lapses are intermittent, rather than constant.
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