The nurse taking Mr. M's vitals asks him about his tiredness. "Do you ever have trouble balancing or walking? Do you drop things? And have you noticed any tingling or cramping?"
"Funny you should ask," he says. "Last week I was having all kinds of foot cramps and twitches. But this week, that's all gone. Sometimes at night I'm so tired I can hardly stand up. And I just told my daughter I had the dropsy - I drop everything I touch."
"Well, let's check your reflexes." Mr. M's reflexes are slow and weak.
Of all the things the nurse could have checked, why reflexes? When you ask her, she says "That story about cramps and twitches last week and his weakness today - I'm worried about his potassium levels. With renal failure, he can't clear it out of his blood properly."
But what does potassium have to do with cramps and reflexes? Let's review the normal process of nerve and skeletal muscle firing.
Here is a graph of nerve firing. Match the descriptions of what is happening into the proper places on the graph.