Select the topic sentence.

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

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