1. An apparatus comprising: a threshold voltage adjustment circuit configured to perform an operation configured to change a threshold voltage of a first select transistor and configured to maintain a threshold voltage of a second select transistor that is adjacent to the first select transistor; and a defect determination circuit configured to flag the first select transistor and the second select transistor in response to the threshold voltage of one or more of the first select transistor and the second select transistor failing to meet a target threshold voltage in response to the operation.