**Core Concept**
Case-control studies are a type of observational study design used in epidemiology to investigate the association between a particular exposure and an outcome or disease. They involve selecting participants who have the disease (cases) and comparing them to participants without the disease (controls) to identify potential risk factors.

**Why the Correct Answer is Right**
In case-control studies, the odds ratio (OR) can be calculated to estimate the strength of the association between the exposure and outcome. This is because case-control studies are retrospective, meaning they look back in time to identify participants who have developed the disease. The OR is calculated as the ratio of the odds of exposure among cases to the odds of exposure among controls. This allows researchers to quantify the magnitude of the association between the exposure and outcome.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because case-control studies do not proceed from cause to effect. Instead, they identify potential risk factors and then look back in time to determine whether these factors were present in cases and controls.

**Option C:** This option is incorrect because incidence cannot be calculated in case-control studies. Incidence refers to the number of new cases of a disease that occur over a specified period of time, and case-control studies are not designed to measure incidence.

**Option D:** This option is incorrect because case-control studies do not necessarily require many patients. While larger sample sizes can provide more precise estimates of the odds ratio, case-control studies can be conducted with relatively small sample sizes, especially if the exposure and outcome are strongly associated.

**Clinical Pearl / High-Yield Fact**
One key limitation of case-control studies is that they are prone to recall bias, where participants may misremember their exposure history. To minimize this bias, researchers typically use multiple sources of information, such as medical records and interviews, to verify exposure status.

**✓ Correct Answer: B. Odds ratio can be calculated**