Final answer:
The nurse should report the finding of a client who is on lamotrigine and has developed a rash, as it may indicate a serious and potentially life-threatening skin reaction.
Step-by-step explanation:
The nurse should report the finding of a client who is taking lamotrigine and has developed a rash. Lamotrigine can cause a serious skin reaction that could be life-threatening, particularly if it is the beginning of Stevens-Johnson syndrome or toxic epidermal necrolysis. Therefore, a rash in a client taking lamotrigine necessitates immediate clinical attention and assessment.
As for the other clients' findings: A client with a WBC count of 7,500/mm3 while taking clozapine is within normal range (though they should be monitored for signs of agranulocytosis); a platelet count of 150,000/mm3 for a client on valproate is also within normal limits; and a lithium level of 1.2 mEq/L is at the upper end of the therapeutic range for a client taking lithium, requiring monitoring but not necessarily immediate reporting unless symptoms of toxicity are present.