Every FMK pathway as a clickable map. Pick one pathway to hammer it — next step, enzyme, regulator, cofactor, and the facts that get tested (NADPH and who needs it, rate-limiting steps, deficiencies) — or set it to all pathways for a mixed drill. Every answer says which enzyme and which pathway.
Pathway
Heme synthesis (porphyrias) · Mitochondria (steps 1, 6–8) and cytosol (steps 2–5); ~85% in marrow erythroid precursors, the rest mostly liver (P450 turnover). Enzymes sit beside the arrows, ★ = rate-limiting, ⊕/⊖ = activators/inhibitors (toggle), −ATP+NADH = what the step spends and makes (toggle ⚡), dashed = allosteric signal, shaded bands = compartment. Drag to pan.
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Why the ring matters
Four pyrroles, joined in a line, then closed
Porphobilinogen is a single pyrrole. Four of them are strung together head-to-tail into a linear chain, and only then closed into a macrocycle. That closure is the line the whole classification hangs on: what accumulates before it is linear and cannot absorb light, and what accumulates after it is a ring that can.
Deficiency: Erythropoietic protoporphyria (marrow origin) — painful, burning photosensitivity WITHOUT frank blistering from childhood; protoporphyrin in RBCs and skin; hepatobiliary disease from biliary protoporphyrin in severe cases
Inner mitochondrial membrane. When iron can't be inserted, zinc is — zinc protoporphyrin is the lab marker.