This thread is directed at those taking USMLE boards. I will try to put together some tough and tricky ones that determine who get's an average vs top score. All members can contribute to this thread. No "run of thre mill" stuff will be included (stuff that u will find in every mcq set ). Otherwise it will be a waste of my time and yours. Just an attempt to make the D-day the sweetest (-:
The small town doctor in the movie Doc Hollywood impresses the aspiring plastic surgeon by treating a cyanotic child with a soft drink, knowing the child has reduced hemoglobin from drinking well water. The ability of hemoglobin to serve as an effective transporter of oxygen and carbon dioxide between lungs and tissues is explained by which of the following properties? A. The isolated heme group with ferrous iron binds oxygen much more avidly than carbon dioxide. B. The α ”“ and β - globin chains of hemoglobin have very different primary structures than myoglobin. C. Hemoglobin utilizes oxidized ferric iron to bind oxygen, in contrast to the ferrous ion of myoglobin. D. In contrast to myoglobin, hemoglobin exhibits greater changes in secondary and tertiary structure after oxygen binding. E. Hemoglobin binds proportionately more oxygen at low oxygen tension than does myoglobin. EXPLANATION: Successive binding of oxygen atoms to hemoglobin progressively changes the tertiary and quaternary structure to produce cooperative kinetics and a sigmoidal oxygen-binding curve. This cooperativity does not occur with the subunits of myoglobin despite similar primary and secondary protein structure (incorrect answer b). Hemoglobin thus binds proportionately less oxygen within the capillaries of low-oxygen peripheral tissues and allows more oxygen delivery to these tissues (incorrect answer e). Isolated heme binds carbon dioxide 25,000 times more strongly than oxygen, but in myoglobin and each hemoglobin chain, a histidine group interferes with the preferred mode of carbon dioxide binding such that oxygen is favored (incorrect answer a). Oxidation of the ferrous iron in myoglobin or hemoglobin to ferric ion abolishes oxygen binding, in contrast to the case with other proteins like cytochromes or catalase, where oxidation/reduction of iron modulates their function (incorrect answer c). Nitrates in well water inhibit methemoglobin reductase and increase the percentage of hemoglobin with ferric ion (methemoglobin). Some mutant hemoglobins like hemoglobin M stabilize the ferric ion and produce more peripheral cyanosis. The answer is D xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx Comments - Here the stem of the question has little to do with what is being asked. So we will go back and read the question again thinking that we are missing something. This is actually a cruel joke...but that's the way life is sometimes ! Extra Edge Pathophysiology Normally, methemoglobin levels are <1%, as measured by the co-oximetry test. Elevated levels of methemoglobin in the blood are caused when the mechanisms that defend against oxidative stress within the red blood cell are overwhelmed and the oxygen carrying ferrous ion (Fe[SUP]2+[/SUP]) of the heme group of the hemoglobin molecule is oxidized to the ferric state (Fe[SUP]3+[/SUP]). This converts hemoglobin to methemoglobin, resulting in a reduced ability to release oxygen to tissues and thereby hypoxia. This can give the blood a bluish or chocolate-brown color. Spontaneously formed methemoglobin is normally reduced (regenerating normal hemoglobin) by protective enzyme systems, e.g., NADH methemoglobin reductase / cytochrome-b5 reductase / Diaphorase 1 (major pathway), NADPH methemoglobin reductase (minor pathway) and to a lesser extent the ascorbic acid and glutathione enzyme systems. Disruptions with these enzyme systems lead to the condition. Hypoxia occurs due to the decreased oxygen-binding capacity of methemoglobin, as well as the increased oxygen-binding affinity of other subunits in the same hemoglobin molecule which prevents them from releasing oxygen at normal tissue oxygen levels. Congenital methemoglobinemia Due to a deficiency of the enzyme diaphorase I (NADH methemoglobin reductase), methemoglobin levels rise and the blood of met-Hb patients has reduced oxygen-carrying capacity. Instead of being red in color, the arterial blood of met-Hb patients is brown. This results in the skin of Caucasian patients gaining a bluish hue. Hereditary met-Hb is caused by a recessive gene. If only one parent has this gene, offspring will have normal-hued skin, but if both parents carry the gene there is a chance the offspring will have blue-hued skin. Another cause of congenital methemoglobinemia is seen in patients with abnormal hemoglobin variants such as hemoglobin M (HbM), or hemoglobin H (HbH), which are not amenable to reduction despite intact enzyme systems. Acquired methemoglobinemia Exposure to exogenous oxidizing drugs and their metabolites (such as benzocaine, dapsone and nitrates) may accelerate the rate of formation of methemoglobin up to one-thousandfold, overwhelming the protective enzyme systems and acutely increasing methemoglobin levels. Other classical drug causes of methemoglobinaemia include antibiotics (trimethoprim, sulfonamides and dapsone), local anesthetics (especially articaine and prilocaine), and others such as aniline dyes, metoclopramide, chlorates and bromates. Ingestion of compounds containing nitrates (such as the patina chemical bismuth nitrate) can also cause methemoglobinemia. Infants under 6 months of age are particularly susceptible to methemoglobinemia caused by nitrates ingested in drinking water, dehydration usually caused by gastroenteritis with diarrhea, sepsis, and topical anesthetics containing benzocaine or prilocaine. Nitrates used in agricultural fertilizers may leak into the ground and may contaminate well water. The current EPA standard of 10 ppm nitrate-nitrogen for drinking water is specifically designed to protect infants. Benzocaine applied to the gums or throat (as commonly used in baby teething gels) can cause methemoglobinemia. Ref - All the above matter has been picked from Wiki - Methemoglobinemia - Wikipedia, the free encyclopedia Coming Back to our case What the small town doc gave to the child was either methylene blue solution ( although iv is more appropriate in this scenario ) or a citus juice containing Vit C ( although Vit C is not useful in such a acute scenario ) What about G6PD deficient patients ? Are they prone to methemoglobinemia ? now this is in my own words in an attempt to keep things simple (-8 Our RBCs are under constant oxidative stress due to the fact that they ferry oxygen. So major proteins and enzymes need to be protected from oxidative stress especially becos our RBCs can't regenerate the proteins becos of a lack of nucleus. As already mentioned we have some dedicated mechanisms to take care of the very precious Hemoglobin and it involves NADH as a cofactor ( major pathway ) . Glutathione and NADPH has aminor role. But the Glutothione system ( using NADPH as a co-factor ) has a major role in protecting the other proteins which includes major membrane proteins. So when a patient with G6PD deficiency faces an oxidative stress...yes he will have methemoglobinemia, but what lands the patient in a crisis is the overwhelming damage to other cellular structures within the RBC ( both membrane and cytoplasmic ) - resulting in hemolysis. So in Methemogloinemis u willhave some peripheral cyanosis and in G6PD def. cyanosis + clogged kidneys >?>?.. And what about pyruvate kinase def ? well these patients have chronic methhemoglobinemia becos of impaired production of NADH ”“ the essential cofactor for diaphorase I. and to make matters worse - these patients are prone to hemolysis ( RBCs are swollen like in hereditary spherocytocis ) :s .this is becos - Erythrocytes manufacture ATP through glycolysis. A deficiency in pyruvate kinase, the enzyme that potentiates the last step of glycolysis (phosphoenolpyruvate converted to pyruvate), results in red blood cells (RBCs) with decreased energy. Lack of ATP impairs the Na+/K+-ATPase and other ATP-dependent processes, leading to a cellular loss of K+ and water and an intracellular accumulation of Na+. the rest u guys know wink)
Wow , Really It Is A Greaaaaaaaaaat Topic , and it is very helpful for any one studying hematology not only for USMLE takers.
