Final answer:
The QRS complex in MCL1 can be negative and still be normal in a third-degree block.
Step-by-step explanation:
The statement that the QRS complex in MCL1 can be negative and still be normal is TRUE. In a third-degree block (also known as complete heart block), there is no correlation between atrial activity (the P wave) and ventricular activity (the QRS complex). This means that the QRS complex can appear negative in lead MCL1 and still be considered normal.
For example, in some cases of complete heart block, the ventricles may be depolarized from a site other than the normal bundle of His-Purkinje system, resulting in a negative QRS complex in MCL1. However, this is still considered normal as long as other parameters of the ECG are within the expected range.