biomedical sub test[kalplan 2001] Discussion is welcome!

zhangheng1020UID: 2130513

A Guatemalan child with a history of meconium ileus is brought in to a clinic because of a chronic cough. The mother notes a history of respiratory tract infections and bulky, foul-smelling stools. After assessment of the respiratory tract illness, the practitioner should also look for signs of
                 A. cystinuria
                 B. hypoglycemia
                 C. iron deficiency anemia
                 D. sphingomyelin accumulation
                 E. vitamin A deficiency

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Explanation:
         The correct answer is E. The child is likely suffering from cystic fibrosis. In this disorder, an abnormality of chloride channels causes all exocrine secretions to be more viscous than normal. Pancreatic secretion of digestive enzymes is often severely impaired, with consequent steatorrhea and deficiency of fat-soluble vitamins, including vitamin A.
         Cystinuria (choice A) is a relatively common disorder in which a defective transporter for dibasic amino acids  (cystine, ornithine, lysine, arginine; COLA) leads to saturation of the urine with cystine, which is not very soluble in urine, and precipitates out to form stones.
         Hypoglycemia (choice B) is not a prominent feature of children with cystic fibrosis who are on a normal diet. Hyperglycemia may occur late in the course of the disease.
         Iron deficiency anemia (choice C) is not found with any regularity in children with cystic fibrosis.
         Sphingomyelin accumulation (choice D) is generally associated with deficiency of sphingomyelinase, as seen in Niemann-Pick disease.

zhangheng1020UID: 2130513

In which of the following laboratory tests would you expect to find the greatest disparity in reference intervals between men and (non-pregnant) women?
                 A. Mean corpuscular volume
                 B. Serum alkaline phosphatase
                 C. Serum ferritin
                 D. Serum glucose
                 E. Serum sodium

zhangheng1020UID: 2130513

Explanation:
         The correct answer is C. Men have higher reference intervals than women in tests related to iron and hemoglobin (Hb) concentration in blood. The normal reference interval for Hb concentration in women is lower (12.0-16.0 gm/dL) than that for men (13.5-17.5 gm/dL) due to lower serum testosterone levels (testosterone is higher in men and stimulates erythropoiesis) and blood loss during menses. Furthermore, women normally have about 400 mg of iron (as ferritin) in their bone marrow iron stores versus an average of 1000 mg of iron for men. In the absence of inflammation, the small circulating fraction of ferritin (choice C) correlates well with ferritin stores in the bone marrow. Hence, men have different reference intervals for serum ferritin than do women (15-200 ng/mL in men versus 12-150 ng/mL in women).
         The mean corpuscular volume (choice A), serum alkaline phosphatase (choice B), serum glucose (choice D), and serum sodium (choice E) are similar in both sexes.

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A competitive inhibitor of an enzyme will
                 A. alter the Vmax of the reaction
                 B. bind to the same site as the substrate
                 C. decrease the apparent Km for the substrate
                 D. decrease the turnover number
                 E. form an irreversible complex with the enzyme

zhangheng1020UID: 2130513

Explanation:
         The correct answer is B. Substances that reduce the activity of an enzyme are called inhibitors. Reversible inhibitors bind to an enzyme but rapidly dissociate from it [in contrast to irreversible inhibitors (choice E), which bind tightly and dissociate very slowly from the enzyme]. There are several types of reversible inhibitors: Competitive inhibitors usually resemble the substrate and compete with it for binding at the active site (choice B). Thus, increasing the concentration of substrate will decrease the percent inhibition of the enzyme. The Vmax is unchanged, but the Km is increased.
         A noncompetitive inhibitor binds with equal affinity to both enzyme and enzyme-substrate complex. This binding leads to a distortion of the substrate binding site, so new substrate cannot bind and/or the product cannot be released. In this kind of inhibition, the Vmax is decreased (choice A), but the Km is not altered. Adding more substrate will not reverse this type of inhibition. This is the equivalent of decreasing the turnover number (choice D).
         An uncompetitive inhibitor does not bind to free enzyme, but binds to the enzyme-substrate complex at a site other than the catalytic site. Once bound by the inhibitor, the enzyme is trapped in the enzyme-substrate complex state until the inhibitor dissociates. In this kind of inhibition, the slope of the reaction (which is the ratio Km/Vmax) remains the same, but both Vmax (choice A) and Km (choice C) are reduced.

zhangheng1020UID: 2130513

A 9-year-old child in a developing country is brought to a clinic by his parents because he has trouble keeping up with his classmates on the playground. Physical examination is remarkable for pulmonary rales. Chest x-ray shows biventricular dilation of the heart. Deficiency of which of the following vitamins is the most likely cause of this child's condition?
                 A. Ascorbic acid
                 B. Retinol
                 C. Riboflavin
                 D. Thiamine
                 E. Vitamin K

zhangheng1020UID: 2130513

Explanation:
         The correct answer is D. Thiamine deficiency is most frequently encountered in alcoholics and in developing countries. Deficiency of this vitamin can take several forms: dilated cardiomyopathy (“wet beriberi ”), polyneuropathy (“dry beriberi”), and mamillary body degeneration (Wernicke-Korsakoff syndrome).
         Ascorbic acid (choice A, Vitamin C) deficiency causes scurvy, associated with capillary fragility, bony abnormalities, and poor wound healing.
         Retinol (choice B, Vitamin A) deficiency causes blindness and impaired immune responses.
         Riboflavin ( choice C) deficiency causes cheilosis, glossitis, and dermatitis.
         Vitamin K (choice E) deficiency causes impaired blood clotting because of decreased production of factors II, VII, IX, and X.

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A 2-month-old child is evaluated for failure to thrive. As the pediatrician is examining the child, a convulsion occurs. Stat serum chemistries demonstrate severe hypoglycemia, hyperlipidemia, lactic acidosis, and ketosis. Physical examination is remarkable for hepatomegaly, a finding confirmed by CT scan, which also reveals  renomegaly. 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 don't 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

An individual with megaloblastic anemia is found to have a significant folate deficiency. Erythropoiesis is hampered in this man due to his inability to perform which type of enzymatic reaction?
                 A. Acyl transfer
                 B. Carboxylation
                 C. Decarboxylation
                 D. Hydroxylation
                 E. Methylation

zhangheng1020UID: 2130513

Explanation:
         The correct answer is E. Folic acid is a pteridine vitamin that exists as tetrahydrofolate (TH4) in its most reduced form. TH4 can accept methyl, methylene, or formyl carbons and transfer them as methyl groups. This function is vital in nucleotide and amino acid synthesis.
         Pantothenic acid is a key vitamin in acyl transfer reactions (choice A). It forms part of coenzyme A, which transfers acyl groups in thiol esters as acetyl CoA, succinyl CoA, and other acyl CoA forms.
         Important vitamins in carboxylation reactions (choice B) include biotin and vitamin K. Biotin carries the carboxyl group in the pyruvate carboxylase and acetyl CoA carboxylase reactions, and vitamin K is utilized in post-translational carboxylation of amino acid residues in blood clotting factors.
         Oxidative decarboxylation reactions (choice C) require thiamine (vitamin B1). Examples include the pyruvate dehydrogenase and alpha-ketoglutarate dehydrogenase complexes.
         Ascorbic acid (vitamin C) is a coenzyme in the hydroxylation (choice D) of lysyl and prolyl residues of collagen.

zhangheng1020UID: 2130513

A 10-year-old child is suspected of having pellagra because of chronic symptoms including diarrhea, a red scaly  rash, and mild cerebellar ataxia. However, his diet is not deficient in protein and he appears to be ingesting adequate amounts of niacin. A sister has a similar problem. Chemical analysis of his urine demonstrates large amounts of free amino acids. Which of the following is the most likely diagnosis?
                 A. Alkaptonuria
                 B. Carcinoid syndrome
                 C. Ehlers-Danlos syndrome
                 D. Hartnup's disease
                 E. Scurvy

zhangheng1020UID: 2130513

Explanation:
         The correct answer is D. The child has Hartnup's disease. This condition clinically resembles pellagra ("diarrhea, dementia, and dermatitis"), and may be misdiagnosed as this nutritional (niacin) deficiency. In fact, niacin therapy may actually be helpful in controlling the symptoms. The underlying problem is a defect in the epithelial transport of neutral amino acids, including tryptophan, which can act as a precursor of niacin. The defective amino acid transport leads to poor absorption of dietary amino acids as well as excess amino acid secretion in the urine.
         Alkaptonuria (choice A) is characterized by urine that turns black upon standing and a debilitating arthritis.
         Carcinoid syndrome (choice B) is seen in patients with carcinoid tumor. It is characterized by episodes of flushing, diarrhea, hypertension, and bronchoconstriction.
         Ehlers-Danlos syndrome (choice C) is a disease characterized by abnormal collagen formation leading to very elastic skin, joint problems, and fragility of some blood vessels and the intestines.
         Scurvy (choice E) is due to vitamin C deficiency. It is characterized by easy bruising and gum problems.

zhangheng1020UID: 2130513

Which of the following amino acids is most responsible for the buffering capacity of hemoglobin and other proteins?
                 A. Arginine
                 B. Aspartic acid
                 C. Glutamic acid
                 D. Histidine
                 E. Lysine