Learn / FMK
Every FMK condition
299 diseases, deficiencies, toxicities and drug mechanisms harvested from every FMK deck — the ones sprinkled through a single bullet as much as the ones with their own slide. Pick what you're given and what you have to name, or let it cycle; distractors come from the same category so you have to discriminate (hypoketotic vs. ketotic hypoglycemia, which GSD, which urea-cycle enzyme…). Every answer shows the whole card with the lecture and slide.
Amino acid · FMK 07.2
Homocystinuria (cystathionine β-synthase deficiency) Autosomal recessive
Defect / target: Cystathionine β-synthase (B6/PLP-dependent transsulfuration enzyme: homocysteine + serine → cystathionine)
Mechanism: Homocysteine cannot enter transsulfuration, so it accumulates (spilling into urine) and is remethylated to excess methionine while downstream cysteine is starved; homocysteine damages connective tissue and endothelium.
↑ Accumulates: Homocysteine (plasma and urine), methionine· ↓ Deficient: Cysteine (low plasma), cystathionine
Presentation: Lens dislocation (ectopia lentis, downward), marfanoid habitus, skeletal abnormalities, intellectual disability/developmental delay, thrombosis (leading cause of death)
Labs: High urinary and plasma homocysteine, high plasma methionine, low plasma cysteine
Treatment: Methionine restriction + folate and vitamin B12 supplementation (supports remethylation escape route); textbook: B6 in responsive forms, cysteine supplementation
Name ≠ mechanism: enzyme defect with systemic disease (vs cystinuria transporter, cystathioninuria benign); lens down, thrombosis
Learn the mechanism: Methionine cycle & homocysteine's two fates →
FMK 07.2 Amino Acid Metabolism II · slide 07.2: 13, 15; 07.1: 28, 31, 32