Final answer:
The nurse's priority instruction is to avoid measuring the client's temperature rectally due to the heightened risk of bleeding in a patient with a low platelet count caused by chemotherapy.
Step-by-step explanation:
Regarding the nurse's priority instruction for measuring vital signs for a client with a low platelet count due to chemotherapy, the correct answer is: A. Don't measure the client's temp rectally. This precaution is necessary because rectal temperature measurement can cause trauma to the rectal mucosa, which is a significant risk when a patient has thrombocytopenia (a low platelet count) as it could lead to bleeding.
The other options are standard procedures for measuring vital signs, which are also important but do not pose the same level of risk to a patient with a low platelet count.