**Core Concept**
The specificity of a diagnostic test, such as ECG in this case, refers to its ability to correctly identify those without the disease (true negative rate). It is calculated as the number of true negatives divided by the sum of true negatives and false positives.

**Why the Correct Answer is Right**
To calculate the specificity of ECG, we need to identify the true negatives (patients without myocardial infarction who have a negative ECG result) and false positives (patients without myocardial infarction who have a positive ECG result). From the given data, there are 171 patients without myocardial infarction who have a negative ECG result (true negatives) and 9 patients without myocardial infarction who have a positive ECG result (false positives). Therefore, the specificity of ECG is calculated as the number of true negatives divided by the sum of true negatives and false positives: 171 / (171 + 9) = 171 / 180.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option does not make sense in the context of specificity calculation, as it would require dividing by a different denominator.
* **Option B:** This option incorrectly calculates specificity by dividing by the total number of patients without myocardial infarction (180), rather than the sum of true negatives and false positives (180).
* **Option C:** This option incorrectly calculates specificity by using the total number of patients with myocardial infarction (520) as the denominator, rather than the sum of true negatives and false positives.

**Clinical Pearl / High-Yield Fact**
When calculating specificity, it's essential to identify the true negatives and false positives correctly, as these values will directly impact the calculated specificity. A high specificity indicates that the test is good at ruling out the disease, but may not be as effective in ruling in the disease.

**Correct Answer: D. 171 / 180.**