Okay, let's tackle this question about incidence calculation. The user wants me to explain the correct answer and why the others are wrong. First, I need to recall the definitions of incidence and prevalence. Incidence is the number of new cases in a specific time period divided by the population at risk during that period. Prevalence, on the other hand, is the total number of existing cases (both new and existing) divided by the total population.

So the question is asking which measure is calculated by new cases divided by population at risk. The correct answer would be incidence. Now, looking at the options, even though they aren't provided here, the correct answer is likely to be the one that mentions new cases over population at risk. The distractors might include prevalence (which uses existing cases), incidence rate (which might use person-time), or maybe some other terms like cumulative incidence versus incidence rate.

For the incorrect options: Prevalence uses existing cases, incidence rate uses person-time, and maybe there's a confusion between incidence and attack rate, which is similar but often used in outbreaks. I need to make sure each wrong option is explained clearly. The clinical pearl should emphasize that incidence is about new cases, which helps in understanding disease emergence or risk factors. Also, remembering that incidence is different from prevalence is key for exams. Let me structure the explanation with the required sections and keep it concise within the character limit.

**Core Concept**
Incidence measures the *risk of developing a new condition* within a defined population over a specific time period. It is calculated as **new cases / population at risk** during that period, distinct from prevalence (total existing cases).

**Why the Correct Answer is Right**
Incidence quantifies the *rate of new disease occurrence* in a population at risk. For example, if 10 new cases of measles occur in a year in a population of 10,000 unvaccinated individuals, the incidence is 10/10,000 (0.1%). This reflects the probability of contracting the disease, not the total burden (which is prevalence).

**Why Each Wrong Option is Incorrect**
**Option A:** *Prevalence* includes both new and existing cases, not just new ones.
**Option B:** *Mortality rate* measures deaths from a disease, not new cases.
**Option C:** *Attack rate* is a type of incidence used in outbreaks but still requires dividing new cases by the population at risk.

**Clinical Pearl / High-Yield Fact**
Never confuse incidence with prevalence—**incidence = new cases / at-risk population**, while **prevalence = total cases / total population**. A high prevalence may indicate either a high incidence or a low recovery/cure rate.

**Correct Answer: C. New cases divided by population at risk**

✓ Correct Answer: B. Coho study