Discussion Guide — Chapter 3

1. "Your body can't tell the difference between a hormone you made and a hormone you were given. Why not? Could it, in principle?"

Listen for: the realization that the molecules are chemically identical and there is no property to detect. Then push: could evolution have built a sensor that distinguishes them? The answer — there was never any selective pressure, because nothing supplied hormones from outside until pharmacology did — is a nice small lesson about why bodies are not designed for the situations medicine puts them in.

2. "A GnRH agonist is used to suppress the reproductive axis. Explain that to someone who just learned what an agonist is."

The best diagnostic question in the chapter. Listen for: whether students reach for delivery pattern rather than for something about the molecule. Students who try to explain it through receptor properties alone have not got §3.5 and will struggle in Chapter 15.

3. "Is 'stimulate your own production instead of replacing it' a good idea?"

Listen for: a genuine two-sided discussion. The mechanistic argument is real and students should be able to state it fairly — preserving the axis, avoiding gland atrophy, keeping physiological regulation. Then the counterarguments: feedback still opposes it, the signal pattern may be wrong, and none of it establishes an outcome. Do not let this collapse into "peptide marketing is nonsense." The argument is reasonable and unproven, and those are different things — that distinction is the most important thing Part III will ask of them.

4. "Cadaver growth hormone was natural. Recombinant growth hormone is engineered in bacteria. One transmitted a fatal disease."

Listen for: students working out that natural/synthetic is orthogonal to safety. Also listen for the overcorrection ("so engineered is safer") — it isn't, in general; the axis simply doesn't predict. Then ask the harder question: what does predict? (What is actually in the preparation, how it was made, what could contaminate it, whether anyone checked.) That list is Chapter 34's syllabus.

5. "The incubation period for prion disease was decades. What does that mean for a compound that's been in use for five years?"

Uncomfortable and worth it. Listen for: the distinction between "no reported harm" and "shown to be safe" — the same distinction Chapter 17 will need. Also listen for nihilism ("so we can never know anything"), which is the wrong lesson; the right one is that safety confidence is a function of exposure-years and follow-up duration, and those are measurable.

6. "Forty years of publicly funded gut hormone physiology produced a hundred-billion-dollar drug class. Nobody in 1975 knew that. How should that affect how we fund research?"

Listen for: students noticing that the argument cuts both ways — you cannot use this to justify any particular line of research, only research in general, because the successes are only identifiable afterward. The strongest version of the counterargument (opportunity cost is real; unfalsifiable "it might pay off later" defenses are available to any project) deserves airing. Flag that Chapter 12 will complicate the happy version of this story considerably.