**Core Concept**
Widmark's formula is a mathematical equation used to estimate the blood alcohol concentration (BAC) in a person's body, taking into account factors such as sex, weight, and the amount of alcohol consumed. This formula is based on the principle that the body metabolizes alcohol at a relatively constant rate, regardless of the initial amount consumed.

**Why the Correct Answer is Right**
Widmark's formula is specifically designed to calculate the BAC in grams per 100 milliliters of blood (g/100ml) based on the amount of alcohol consumed, the individual's sex and weight, and the elapsed time since consumption. The formula involves the calculation of the body's "water space" and the amount of alcohol in that space. This allows for an estimation of the BAC, which can be useful in assessing the level of intoxication and making decisions about patient care.

**Why Each Wrong Option is Incorrect**
* **Option A:** Widmark's formula is not used to measure the level of other substances such as cocaine or opioids; it is specifically designed for alcohol.
* **Option B:** While the formula does take into account the individual's sex and weight, it is not primarily used to estimate their total body water or fluid status.
* **Option C:** The formula is not used to calculate the rate of alcohol metabolism or the time it takes for the body to eliminate alcohol.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that Widmark's formula is an estimate and should be used in conjunction with clinical judgment and other diagnostic tools to assess a patient's level of intoxication and make informed decisions about their care.

**Correct Answer: D. Blood alcohol concentration (BAC).**