**Core Concept**
Case-control studies are retrospective designs that compare individuals with a disease (cases) to those without (controls) to assess exposure history. They are primarily used to identify potential risk factors, not to measure disease frequency directly.

**Why the Correct Answer is Right**
In case-control studies, the odds ratio (OR) is calculated by comparing the odds of exposure among cases and controls. The OR estimates the association between exposure and outcome, even though the study does not follow individuals over time. This is a key strength of case-control designs, especially when incidence data are unavailable.

**Why Each Wrong Option is Incorrect**
Option A: Case-control studies proceed from effect to cause, not cause to effect. This is a fundamental flaw in understanding study design.
Option C: Incidence cannot be calculated in case-control studies because they do not track new cases over time. Incidence requires prospective follow-up.
Option D: Case-control studies do not require large numbers of patients; they rely on existing cases and controls, making them efficient for rare diseases.

**Clinical Pearl / High-Yield Fact**
Remember: **Case-control → Odds ratio; Cohort → Relative risk**. Always distinguish between incidence and prevalence in study designs. Never calculate incidence in case-control studies.

✓ Correct Answer: B. Odds ratio can be calculated