This is because acetyl-CoA is a cofactor for pyruvate carboxylase, the first step in gluconeogenesis.ĥ) Hyperammonaemia can occur with MCAD deficiency because decreased acetyl-CoA -> decreased TCA cycle activity -> decreased ATP -> decreased carbamoyl phosphate synthesis.Ħ) For MCAD deficiency, they might give you a scenario that's very similar to McArdle's, where muscle cramping + myoglobinuria occur following intense exercise, except the catch is that, rather than occurring within minutes, the Sx occur after prolonged exercise, and intramuscular lipid droplets will be seen on biopsy. palmitate), for instance, the answer is LCAD, not MCAD, deficiency.Ĥ) It is the decreased production of acetyl-CoA with carnitine, S-, M- or LCAD deficiencies that causes the hypoglycaemia. So if the question stem mentions the classic hypoketotic/hypoglycaemic vignette, and then tells you there's a build-up of 16C FAs (e.g. Therefore, the answer here would be M- or LCAD deficiency.Ģ) MCAD deficiency is the most common beta-oxidation disorder.ģ) Intestinal fatty acids that are 12 or more carbons in length are absorbed into lymph and bypass the liver. If the latter is present, then that means carnitine can't be deficient, because if it were, then you'd only get fatty acyl-CoA derivatives, but not anything carnitine-related. With all of the practice questions I've done so far, I can say that the USMLE might frame this topic in a few different ways / here are some things you should know:ġ) You'll get a question where they'll tell you the kid's got low blood glucose and is hypoketotic, and then they'll also tell you that muscle biopsy reveals intracellular fatty acyl-carnitines.
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