Final answer:
A highly sensitive test is required when early detection of a rare disease can significantly improve the prognosis, as it minimizes false negatives. In such a scenario, if a patient tests negative on a sensitive test, it is unlikely that they are infected.
Step-by-step explanation:
Scenarios that require a highly sensitive test are ones where the early detection of a disease significantly improves the prognosis or outcome for the patient. Among the options provided, the scenario that fits this requirement is 'B. Rare disease, Early diagnosis improves prognosis'. A highly sensitive test ensures that even low levels of a pathogen or early signs of a disease are detected, minimizing the chances of a false negative.
A false positive occurs when a test indicates a disease or condition is present when it is not. False positives can occur due to cross-reactivity with other substances, technical errors, or the presence of related but non-causative factors. Conversely, a false negative happens when a test fails to detect a disease or condition that is present. Reasons for false negatives can include low levels of the target molecule below the test's detection threshold or technical issues with the test procedure.
If a patient tests negative on a highly sensitive test, the likelihood that the person is actually infected with the pathogen is very low, given the high sensitivity of the test for detecting the disease.