A patient who is wearing a GP lens complains of a "scratching sensation." Which of the following instruments would be most valuable in determining the cause of the discomfort?

The most valuable instrument for determining the cause of a "scratching sensation" experienced by a patient wearing a GP lens is the measuring magnifier. This tool allows the practitioner to closely examine the surface of the lens and check for any debris, scratches, or irregularities that could cause discomfort. By magnifying the lens material, it is possible to detect subtle surface imperfections that might not be visible to the naked eye, thereby helping to diagnose the reason behind the patient's discomfort.

In the context of the other options, while a topogometer is excellent for assessing the curvature and fit of the lens, it does not directly examine surface defects that could cause the scratching sensation. A thickness gauge is used to measure the lens thickness but would not provide insight into the surface irregularities. A radiuscope is helpful for assessing the base curve of the lens but does not assist in identifying surface wear or contaminations that might cause irritation. Therefore, the measuring magnifier is the most appropriate choice for examining the lens's surface qualities that could be responsible for the patient's symptoms.