Final answer:
When a consumer's total utility decreases with the consumption of one additional unit of a good, the marginal utility is negative. This is due to the law of diminishing marginal utility, where each additional unit consumed provides less utility than the previous one.
Step-by-step explanation:
If a consumer's total utility decreases with the consumption of one additional unit of a good, then we know that the marginal utility must be negative. This situation is an example of the law of diminishing marginal utility, which suggests that the additional satisfaction a consumer gets from consuming one more unit of a good decreases with each additional unit consumed. For instance, the first T-shirt a consumer buys may provide a high level of satisfaction (utility), but the fourth T-shirt might not hold as much value since it's just an extra item for when other clothes are unavailable. As such, the marginal utility from the first T-shirt is higher than that from the fourth. When the marginal utility becomes negative, it implies that consuming an additional unit of the good actually reduces the consumer's satisfaction, thus leading to a decrease in total utility.