biomedical sub test[kalplan 2001] Discussion is welcome!

zhangheng1020UID：2130513

A 2-month-old boy is evaluated for failure to thrive. As the pediatrician is examining the patient, she witnesses a seizure. Physical examination is remarkable for hepatomegaly, a finding later confirmed by CT scan, which also reveals renomegaly. Serum chemistries demonstrate severe hypoglycemia, hyperlipidemia, lactic acidosis, and ketosis. Which of the following diseases best accounts for this presentation?
                A. Gaucher's disease
                B. McArdle's disease
                C. Niemann-Pick disease
                D. Pompe's disease
                E. von Gierke's disease

zhangheng1020UID：2130513

Explanation:
         The correct answer is E. von Gierke's disease is a glycogen storage disease caused by a deficiency of glucose-6-phosphatase. It typically presents with neonatal hypoglycemia, hyperlipidemia, lactic acidosis, and ketosis. Failure to thrive is common in early life; convulsions may occur due to profound hypoglycemia. The   glycogen accumulation in von Gierke's disease occurs primarily in the liver and kidneys, accounting for the   enlargement of these organs. Gout may develop later because of the derangement of glucose metabolism.  Even if you do not remember all of the details of the presentation of these genetic diseases, you should be able
         to narrow the choices: Gaucher's disease (choice A) and Niemann-Pick disease (choice C) are lipid storage diseases, and would not be expected to produce hypoglycemia. The other diseases are glycogen storage diseases, but McArdle's (choice B) and Pompe's (choice D) disease affect muscle rather than liver and would not be expected to produce profound hypoglycemia, since the liver is the major source for blood glucose.

zhangheng1020UID：2130513

A newborn appears normal at birth, but develops vomiting and diarrhea accompanied by jaundice and hepatomegaly within the first few weeks of life. Within months, the baby has obvious cataracts and ascites. The infant is switched to a milk-free diet, which stabilizes but does not completely reverse his condition. By one year of age, he has developed mental retardation. Which of the following is the most likely diagnosis?
                A. Cystic fibrosis
                B. Galactosemia
                C. McArdle's disease
                D. Von Gierke's disease
                E. Wilson's disease

zhangheng1020UID：2130513

Explanation:
         The correct answer is B. Galactosemia is an autosomal recessive disease caused by a deficiency of galactose-1-phosphate uridyltransferase, which is necessary for the metabolism of the galactose derived from milk lactose. The condition should be suspected in infants with growth failure, cataracts, liver disease, aminoaciduria, and mental retardation. A reducing sugar (galactose) is usually present in the urine. Most of the  pathology is related to the toxic effects of galactose-1-phosphate. Treatment involves strict dietary lactose restriction, which consists of more than simply withdrawal of milk products, because lactose is also present in many non-diary foods. Strict adherence to the diet can strikingly alter the course of this disease. Cystic fibrosis (choice A) is associated with maldigestion pancreatic disease, and pulmonary disease. McArdle's disease (choice C) is a glycogen storage disease that selectively affects muscle. Von Gierke's disease (choice D) is a glycogen storage disease affecting the liver and kidneys. Wilson's disease (choice E) is a caused by a metabolic abnormality in the handling of copper that can cause  cirrhosis and brain damage, and usually presents in adolescence.

zhangheng1020UID：2130513

35
Which of the following pairs of enzymes is required for the process of gluconeogenesis?
               A. Fructose-1,6-bisphosphatase and pyruvate carboxylase
               B. Glucose-6-phosphatase and phosphofructokinase-1
               C. Glucose-6-phosphatase and pyruvate dehydrogenase
               D. Phosphoenolpyruvate carboxykinase and glucokinase
               E. Pyruvate kinase and pyruvate carboxylase

zhangheng1020UID：2130513

Explanation:
        The correct answer is A.The three irreversible steps of glycolysis are catalyzed by hexokinase, phosphofructokinase-1 (choice B), and pyruvate kinase. In gluconeogenesis, other enzymes are needed to bypass these key steps. Pyruvate cannot be directly converted to phosphoenolpyruvate in gluconeogenesis. Therefore, pyruvate carboxylase (a mitochondrial enzyme; choice A) converts pyruvate to oxaloacetate, which can be converted to phosphoenolpyruvate by phosphoenolpyruvate carboxykinase (choice D), using two ATP equivalents per molecule of phosphoenolpyruvate. Fructose-1,6-bisphosphatase (choice A) is the enzyme that  splits fructose-1,6-bisphosphate into fructose-6-phosphate and inorganic phosphate. It is also required for gluconeogenesis.
        Glucose-6-phosphatase (choices B and C) is a liver enzyme that hydrolyzes glucose-6-phosphate to glucose. A deficiency of this enzyme leads to von Gierke disease, also known as glycogen storage disease type I. Pyruvate dehydrogenase (choice C) is a mitochondrial enzyme that converts pyruvate to acetyl CoA. This enzyme requires thiamine pyrophosphate, lipoamide, and FAD as cofactors.  Glucokinase (choice D) is a liver enzyme that converts glucose to glucose-6-phosphate. Unlike hexokinase, it is specific for glucose and is unresponsive to the level of glucose-6-phosphate. Its function is to store excess glucose, so it has a very high Km (ie, a low affinity) for glucose, becoming active only when the concentration of glucose is very high.
        Pyruvate kinase (choice E) catalyzes the conversion of phosphoenolpyruvate to pyruvate in the glycolytic pathway. It is activated by fructose-1,6-bisphosphate, the product of the committed step of glycolysis, and is allosterically inhibited by ATP, alanine, and acetyl CoA.

zhangheng1020UID：2130513

A 40-year-old, formerly obese woman presents to her physician. She was very proud of having lost 80 lbs. during the previous 2 years, but now noticed that her "hair is falling out." On questioning, she reports having followed a strict fat-free diet. Her alopecia is probably related to a deficiency of which of the following vitamins?
                 A. Vitamin A
                 B. Vitamin C
                 C. Vitamin D
                 D. Vitamin E
                 E. Vitamin K

zhangheng1020UID：2130513

Explanation:
         The correct answer is A. While it is hard to develop a deficiency in oil-soluble vitamins (A, D, E, K) because the liver stores these substances, deficiency states can be seen in chronic malnutrition (specifically chronic fat deprivation) and chronic malabsorption. Vitamin A is necessary for formation of retinal pigments (deficiency can cause night blindness) and for appropriate differentiation of epithelial tissues (including hair follicles, mucous membranes, skin, bone, and adrenal cortex).
         Vitamin C (choice B), which is water soluble rather than oil soluble, is necessary for collagen synthesis.
         Vitamin D (choice C) is important in calcium absorption and metabolism.
         Vitamin E (choice D) is a lipid antioxidant that is important in the stabilization of cell membranes.
         Vitamin K (choice E) is necessary for normal blood coagulation.

zhangheng1020UID：2130513

5 mL of synovial fluid is aspirated from an inflamed knee joint. The fluid is yellow-white and cloudy and contains 200,000 WBC/mm3 (85% neutrophils). Needle-shaped, strongly negatively birefringent crystals are seen both within and outside neutrophils. These crystals most likely have which of the following compositions?
                A. Basic calcium phosphate
                B. Calcium oxalate
                C. Calcium pyrophosphate dihydrate
                D. Cholesterol
                E. Monosodium urate

zhangheng1020UID：2130513

Explanation:
         The correct answer is E. All the compounds listed can produce crystals in joint fluid, but only monosodium urate  (associated with gout) and calcium pyrophosphate dihydrate (associated with CPPD crystal deposition disease, also called pseudogout), and to lesser degree basic calcium phosphate (apatite-associated arthropathy), have a high likelihood of being encountered on a step 1 USMLE exam. The crystals described are those of monosodium urate. Be careful not to answer "uric acid" if that is listed as an alternative choice on an exam, since the sodium salt is the predominant species in vivo.
         Basic calcium phosphate (choice A) is seen in apatite-associated arthropathy and produces spherical clumps of nonbirefringent submicroscopic crystals.
         Calcium oxalate crystals (choice B) are seen in primary oxalosis and are bipyramidal, positively birefringent crystals.
         Calcium pyrophosphate dihydrate crystals (choice C) are a feature of pseudogout and are rod-to-rhomboidal-shaped, weakly positively birefringent crystals.
         Cholesterol crystals (choice D) are seen in chronic and chylous effusions in inflammatory and degenerative arthritis, where they form large, flat, rhomboidal plates with notched corners.

zhangheng1020UID：2130513

38
Which of the following cofactors is required for decarboxylation of alpha-ketoacids?
                 A. Vitamin B1
                 B. Vitamin B2
                 C. Vitamin B3
                 D. Vitamin B5
                 E. Vitamin B6

zhangheng1020UID：2130513

Explanation:
         The correct answer is A. Vitamin B1, or thiamine, is the coenzyme required (as the pyrophosphate) for the decarboxylation of alpha-ketoacids. An example of this reaction is pyruvate decarboxylase reaction in alcoholic fermentation. Other reactions such as that catalyzed by pyruvate dehydrogenase also rely on thiamine   pyrophosphate for decarboxylation, but require other cofactors as well. Thiamine is also required for the  generation of pentose phosphates for nucleotide synthesis in the pentose phosphate pathway (hexose   monophosphate shunt), serving as a cofactor for transketolase.
         Vitamin B2(choice B), or riboflavin, is a constituent of FMN (flavin mononucleotide) and FAD (flavin adenine dinucleotide). It functions in hydrogen and electron transport.
         Vitamin B3(choice C), or niacin (nicotinic acid), is a coenzyme that is also involved in hydrogen and electron transport. Nicotinic acid functions in the form of NAD and NADP.
         Vitamin B5(choice D), or pantothenic acid, is conjugated with coenzyme A to act as a carboxylic acid carrier.
Vitamin B6(choice E), or pyridoxine, is required as a cofactor for pyridoxal phosphate and pyridoxamine phosphate. Both of these cofactors are essential to protein metabolism and energy production.

zhangheng1020UID：2130513

A newborn presents with severe acidosis, vomiting, hypotonia, and neurologic deficits. Serum analysis reveals elevated levels of lactate and alanine. These observations suggest a deficiency in which of the following enzymes?
                A. Alanine aminotransferase
                B. Glutamate dehydrogenase
                C. Lactate dehydrogenase
                D. Pyruvate carboxylase
                E. Pyruvate dehydrogenase

樱桃小丸子/丸丸UID：2160265

Originally posted by zhangheng1020 at 2005-12-1 12:11

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zhangheng1020UID：2130513

Explanation:
         The correct answer is E. Pyruvate dehydrogenase (PDH) catalyzes the irreversible conversion of pyruvate to acetyl-CoA. If PDH is absent, pyruvate will be used in other pathways instead. Pyruvate will be converted to alanine via alanine aminotransferase (choice A) and to lactate via lactate dehydrogenase (choice C).
         Glutamate dehydrogenase (choice B) is involved in oxidative deamination, releasing ammonium ion for urea synthesis. Deficiency of this enzyme would not cause the symptoms described.
         Pyruvate carboxylase (choice D) is a gluconeogenic enzyme that catalyzes the conversion of pyruvate to  oxaloacetate. Deficiency of this enzyme would not cause the symptoms described.