Publish and Perish, part 36: Office hours

[Context: read the previous part or start at the beginning. TW: bad amateur fiction!]

Cissy’s Toto Hypothesis was still on Gerald’s mind the next day when he heard a student approaching his office.

“Come in!” he said. “How’s it going, Shawna?”

“OK,” she said. “I just wanted to ask you about something on the test.” She put down her backpack, pulled out the exam, and settled into his guest chair. “Uh, where was it … Oh, yeah, this one.” She pointed at a short-answer question. “Why did I lose two points here?” She said it in an assertive but polite tone that Gerald appreciated.

“All right, let me start by re-reading your answer,” he said. He took a look, then nodded as he recognized the mistake. It was always a relief to confirm a previous grading judgment, often made late at night. 

“OK,” he said, settling into his gentle-explainer mode. “This question, like all of the short-answer questions, was worth eight points. You got six points out of eight, which means that your answer got something important right, but also missed something important.”

“Your answer,” he continued, “correctly says that an infusion of hypotonic saline would initially raise the patient’s blood volume and thus blood pressure. You said that really well. However, you also said that this infusion would raise plasma osmolarity, which is not true. If the add fluid is lower in osmolarity than the blood, the blood’s osmolarity will go down.”

He looked at Shawna, who looked unconvinced.

“But…” she said. “Saline is a salt solution, right?”

“Right.”

“So…if you add saline to the blood you’re adding more salt to the blood, right?”

“More salt as well as more water, yes.”

“So if you’re adding more salt to the blood, doesn’t the blood’s osmolarity increase?”

“No,” Gerald said, “because you’re adding water to an even greater extent. Let’s back up to an extreme case. What if we infused pure water into the blood? That would dilute the blood and drive its osmolarity down, yes?”

“Um, yes.”

“OK, good. Now imagine that we do this water infusion with just a teeny tiny bit of salt sprinkled into the water. The infusion is now adding a bit of extra solute into the blood, but not enough to offset all of the extra water that’s being added, right?”

Shawna thought about this. “I guess so,” she said.

“The test question is essentially about this scenario. You’re adding a solution that is not pure water but is mostly water, with just a little bit of salt. So the osmolarity of the blood goes down.”

“Yeah, I guess that makes sense.” She paused. “So is dilute saline really used in hospitals?”

“Not often,” said Gerald. “I mean, I can imagine it happening in cases of hypernatremia, but I think those must be pretty rare.”

“So…wasn’t this question sort of weird?”

“I think that’s fair to say,” Gerald allowed. “Some of my questions are rather contrived, to make sure that you can apply your knowledge to unusual situations.”

“But…is it fair to hit us with these weird questions out of the blue on a test?” Her tone was still respectful; the question didn’t feel like an accusation.

Gerald nodded to acknowledge her point. “There’s always a risk that we’ll ask too much of you,” he admitted. “But I try really hard to give you practice problems that illustrate my expectations really clearly. Like, do you remember that worksheet we did about the guy whose fraternity brothers made him drink his own sweat?”

“Was that the one about Kappa Yappa Trappa?”

Gerald smirked at his own ridiculous neologism. “No, I think that was the one about the cranial nerve issue. In this other case…” — he paused to recall his worksheet — “I think I called the fraternity Delta Alpha Two.”

“Oh.”

“Anyway, sweat is, essentially, a form of hypotonic saline. So if you stripped away the frat-boy shenanigans, that problem was very similar to this one.”

“Huh.” Shawna thought that over for a few seconds. “I guess so.” She stood up. “OK, Dr. C, thanks for the explanations. Before the next test I’ll spend more time reviewing those worksheets.”

“Yup, that sounds like a good strategy,” Gerald affirmed. “Anything else?”

“Not really, thanks.”

“Well, thanks for coming by!”

Gerald took a step out of his office, watched her recede down the hallway, and noticed another student heading his way.

[Update: the story continues with part 37.]