Pick a drug, toxin, enzyme inhibitor or mutation. Learnex traces what builds up before the block, what runs dry after it and how the body compensates.
Enter a drug, toxin, enzyme inhibitor or mutation, for example a statin, cyanide, an ACE inhibitor or an insulin receptor defect.
Learnex names the molecular target, such as an enzyme, channel or receptor, and where in the cell it acts.
Each stage is labelled Normal, Inhibited, Accumulating, Depleted or Compensating, with its location, inputs and outputs.
Answer a board-style challenge question about the causal chain and get feedback on your logic.
Paired cards show what accumulates before the block and what is depleted after it.
Normal, Inhibited, Accumulating, Depleted and Compensating make the flow of the cascade easy to read.
Ordered stages with cellular locations, inputs and outputs, and the rate-limiting checkpoints that matter.
Compare negative feedback that restores a set point with positive feedback that amplifies to a climax.
MCAT-style and board-style questions that test whether you can reason through cause and effect.
A statin inhibits HMG-CoA reductase, the rate-limiting enzyme of cholesterol synthesis. Here is how the cascade reads.
| Stage | Status | Why |
|---|---|---|
| HMG-CoA reductase | Inhibited | Direct target of the drug |
| HMG-CoA | Accumulating | Upstream substrate can't be converted as fast |
| Mevalonate and later intermediates, including cholesterol | Depleted | Downstream products are made more slowly |
| LDL receptor expression in the liver | Compensating | Low cell cholesterol switches on SREBP-2, so liver cells take up more LDL from the blood |
Key Takeaway
The compensation step is the exam point: the drug lowers blood LDL partly because the cell responds to its own cholesterol shortage.
The substrate just before the block builds up (upstream accumulation) and the products after it fall (downstream depletion). The cell may partly compensate through feedback regulation or alternative routes. In a pathway A to B to C, blocking the enzyme that converts A to B raises A and lowers B and C.
Upstream accumulation is the backing-up of molecules that can no longer be processed past the block. Downstream depletion is the shortage of everything the blocked step would have made, which starves later processes. The Bio Lab shows both side by side.
Compensating marks a stage where the cell or body responds to the block, for example by changing gene expression, using an alternative pathway or adjusting a feedback loop. Statins, for instance, trigger more LDL receptors on liver cells.
It includes MCAT-style and board-style challenge questions that test causal reasoning about pathways, drugs and mutations. Use it alongside your official course and exam materials.
No. The Bio Lab uses simplified models for education. It is not for diagnosis, treatment or any clinical decision.
You can enter any drug, toxin, enzyme inhibitor or genetic mutation, for example a statin, cyanide, an ACE inhibitor or an insulin receptor defect. Results are AI-generated, so confirm details against your course materials.
Practice the step-by-step logic exams actually test with Learnex Labs.
START FREE →Last updated 2026-11-04