## Core Concept
Cohort studies are a type of observational study design used in epidemiology and medical research. They involve following a group of individuals (the cohort) over time who share a common characteristic, with the goal of assessing how their exposures affect their outcomes. This helps in understanding the development and incidence of diseases.

## Why the Correct Answer is Right
Cohort studies are particularly useful for studying rare exposures and can establish temporality between exposure and outcome. They can be prospective or retrospective. However, without the specific details of options A, B, C, and D, we can infer based on general knowledge of cohort studies. Typically, cohort studies are known for their ability to provide strong evidence for causality, their prospective design allows for the measurement of incidence, and they are useful for studying the etiology of diseases.

## Why Each Wrong Option is Incorrect
- **Option A:** Without specific details, we can't directly address why A is incorrect, but generally, cohort studies are indeed prospective or retrospective, allowing for the measurement of incidence and relative risk.
- **Option B:** Similarly, cohort studies do allow for the assessment of the temporal relationship between exposure and outcome, making them valuable for inferring causality.
- **Option C:** This could potentially be about the cost or efficiency, as cohort studies, especially prospective ones, can be time-consuming and expensive.
- **Option D:** This option is marked as the correct answer but without context, we assume it highlights a limitation or incorrect statement about cohort studies.

## Clinical Pearl / High-Yield Fact
A key point to remember is that cohort studies are particularly useful for studying **rare exposures** and can help establish a **temporal relationship** between exposure and disease, which is crucial for inferring causality.

## Correct Answer: D.