Which of the following is not deductible from the adjusted gross income (AGI) to calculate taxable income?

The standard deduction is a set amount that taxpayers can deduct from their income to reduce their taxable income. Unlike other deductions, which may vary based on expenses or specific calculations, the standard deduction is not itemized and is instead applied directly to the taxpayer's income. This means it is taken off the top when calculating taxable income, rather than being deducted from adjusted gross income (AGI).

On the other hand, self-employment tax and ordinary and necessary business expenses are both subtracted from AGI in order to determine taxable income. Similarly, medical expenses can also be deducted, but only to the extent that they exceed 7.5% of AGI. The standard deduction does not require any calculations related to AGI and is instead a flat deduction, making it the exception among the listed options. Thus, understanding that the standard deduction is a standard reduction rather than a deductible expense helps clarify why it is not included in the process of calculating taxable income from AGI.