biomedical sub test[kalplan 2001] Discussion is welcome!

zhangheng1020UID：2130513

Explanation:
          The correct answer is D. Remember that a buffer is most effective when its pKa is within the pH range of the surrounding medium. Histidine is the only amino acid with good buffering capacity at physiologic pH. The  imidazole side chain of histidine has a pKa around 6.0 and can reversibly donate and accept protons at physiologic pH.
          Arginine (choice A) and lysine (choice E) are basic amino acids with pKa's of 12.5 and 10.5, respectively; at physiologic pH both will behave as bases and accept protons.
          Aspartic acid (choice B) and glutamic acid (choice C) are acidic amino acids with pKa's of approximately 4; at physiologic pH they will behave as acids and donate protons.

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An 8-month-old female child is brought to medical attention because her first four teeth show several discrete, discolored, circumferential bands that show very little enamel. Excessive levels of which of the following may have produced this defect?
                A. Bilirubin
                B. Fluoride
                C. Parathormone
                D. Thyroid hormone
                E. Vitamin C

zhangheng1020UID：2130513

Explanation:
         The correct answer is B. Fluoride excess causes direct injury to ameloblasts, leading to inadequate production of tooth enamel. The resultant defect in enamel production causes recessed and discolored rings on the emerging teeth. Other conditions that cause enamel hypoplasia include inadequate levels of calcium, phosphorus, and vitamins A, C, or D, hypothyroidism, and hypoparathyroidism.
Jaundice in the newborn period may cause bilirubin (choice A) deposition in the developing teeth, resulting in pigmented, but otherwise normal teeth.
         Hyperparathyroidism (choice C), with resultant hypercalcemia, will result in osteoporosis, renal stones, and neuromuscular weakness, but no obvious dental changes. Enamel hypoplasia is associated with hypoparathyroidism.
         Hyperthyroidism (choice D) characteristically produces cardiac and nervous disturbances, weight loss, hypermetabolism, and proptosis, but does not cause enamel hypoplasia. Hypothyroidism, not hyperthyroidism, produces enamel hypoplasia.
         Excess dietary vitamin C (choice E) is generally excreted in the urine and produces no systemic pathology. Scurvy, due to inadequate vitamin C, may produce enamel hypoplasia.

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A chronic alcoholic develops severe memory loss with marked confabulation. Deficiency of which of the following vitamins would be most likely to contribute to the neurologic damage underlying these symptoms?
A. Folic acid
B. Niacin
C. Riboflavin
D. Thiamine
E. Vitamin B12

zhangheng1020UID：2130513

Explanation:
The correct answer is D.  Wernicke-Korsakoff syndrome refers to the constellation of neurologic symptoms caused by thiamine deficiency. Among these, a severe memory deficit, which the patient may attempt to cover by making up bizarre explanations  (confabulation), is prominent. Anatomical damage to the mamillary bodies and periventricular structures has been postulated as the cause. In the U.S., severe thiamine deficiency is seen most commonly in chronic alcoholics.  Thiamine deficiency can also damage peripheral nerves ("dry" beriberi) and the heart ("wet" beriberi).
Folic acid deficiency (choice A) produces megaloblastic anemia without neurologic symptoms.
Niacin deficiency (choice  B) produces  pellagra,  characterized by  depigmenting dermatitis,  chronic diarrhea, and anemia.
Riboflavin deficiency  (choice  C)  produces  ariboflavinosis,  characterized  by  glossitis,  corneal
opacities, dermatitis, and erythroid hyperplasia.
Vitamin B12 deficiency (choice  E) produces  megaloblastic anemia accompanied  by degeneration of the posterolateral spinal cord.

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A  25-year-old woman with  sickle cell anemia complains of steady pain in her right upper quadrant with radiation  to the  right shoulder,  especially  after large  or  fatty meals.  Her physician  diagnoses gallstones. Of which of the following compounds are these stones most likely composed?
A. Calcium bilirubinate
B. Calcium oxalate
C. Cholesterol
D. Cholesterol and calcium bilirubinate
E. Cystine

zhangheng1020UID：2130513

Explanation:
The  correct answer  is A.  Bilirubin  is a  degradative  product of  hemoglobin metabolism.  Bilirubin (pigment)  stones  are  specifically  associated  with  excessive  bilirubin  production  in  hemolytic anemias,  including  sickle cell  anemia. Bilirubin  stones can also  be seen  in hepatic cirrhosis and liver fluke infestation.
Calcium  oxalate stones  (choice B) and cystine stones  (choice E) are found in the kidney, rather than the gallbladder.
Pure  cholesterol  stones (choice  C) are  less common  than mixed  gallstones, but  have the same risk factors, including obesity and multiple pregnancies.
Mixed  stones (choice  D)  are the  common  "garden  variety" gallstones,  found  especially in  obese, middle aged patients, with a female predominance.

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Two  sisters are diagnosed with hemolytic anemia. Their older brother was previously diagnosed with the same  disorder.  Two other  brothers  are  asymptomatic. The  mother  and  father are  second  cousins. Deficiency of which of the following enzymes would be most likely to cause this disorder?
A. Debranching enzyme
B. Glucose-6-phosphatase
C. Glucose-6-phosphate dehydrogenase
D. Muscle phosphorylase
E. Pyruvate kinase

zhangheng1020UID：2130513

Explanation:
The  correct  answer  is  E.  In  general,  you should  associate  hemolytic  anemia  with  defects  in glycolysis  or  the hexose monophosphate  shunt (pentose phosphate pathway).  Only two enzymes of those listed  in  the  answer  choices  specifically  involve these  pathways  and  cause  hemolytic  anemia: pyruvate   kinase  and  glucose-6-phosphate  dehydrogenase.  Glucose-6-phosphate  dehydrogenase  (G6PD) deficiency  is  inherited as an  X-linked recessive  trait, so females  would not be affected. Pyruvate kinase  is  a  glycolytic enzyme;  pyruvate  kinase  deficiency  is  an autosomal  recessive  disorder, affecting  males and  females  approximately equally.  If  this  enzyme is  deficient,  red cells  have trouble  producing enough  ATP to maintain the  Na+/K+ pump on the plasma membrane, secondarily causing swelling and lysis.
Debranching enzyme (choice A) defects produce Cori's disease, one of the glycogen storage diseases.
Defects  in  glucose-6-phosphatase  (choice B)  produce  Von  Gierke's  disease,  one of  the  glycogen storage diseases.
Glucose-6-phosphatase dehydrogenase (choice C) deficiency produces an X-linked hemolytic anemia.
Defects  in  muscle phosphorylase  (choice D)  produce McArdle's  disease, one  of the glycogen storage diseases.

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Which of the following amino acids would most likely be found on the surface of a protein molecule?
A. Alanine
B. Arginine
C. Isoleucine
D. Leucine
E. Phenylalanine
F. Tryptophan

zhangheng1020UID：2130513

Explanation:
The  correct  answer is  B. This  question requires  two logical  steps: first,  you need to appreciate that  the  hydrophilic amino  acids are  more likely  to appear on  the surface  of a protein molecule, while  hydrophobic amino  acids are most likely be found  in its interior. Next, you need to figure out which  of  the amino acids  listed is hydrophilic.  If you  recall that arginine  is a basic amino acid that is positively charged at physiologic pH, you should be able to answer this question right away.
All  of  the other  choices have neutral  side chains and  are uncharged  at physiologic pH. They would most  likely  be  found  in  the  hydrophobic  core of  the  protein  structure.  Alanine  (choice  A), isoleucine  (choice  C), and leucine  (choice D) all have  aliphatic side chains; phenylalanine (choice E) and tryptophan (choice F) have aromatic side chains.

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A Southeast Asian immigrant child is noted to be severely retarded. Physical examination reveals a pot-bellied,  pale  child with a  puffy face. The  child's tongue is  enlarged. Dietary deficiency of which of the following substances can produce this pattern?
A. Calcium
B. Iodine
C. Iron
D. Magnesium
E. Selenium

zhangheng1020UID：2130513

Explanation:
The  correct answer  is B.  The  disease is  cretinism, characterized  by  a profound  lack of  thyroid hormone  in  a developing child, leading  to mental retardation  and the physical findings described in the  question  stem. Cretinism  can be due  to dietary deficiency  of iodine  (now rare in this country because  of iodized  salt), to developmental failure of  thyroid formation, or to a defect in thyroxine synthesis.
Calcium deficiency (choice A) in children can cause osteoporosis or osteopenia.
Iron deficiency (choice C) can cause a hypochromic, microcytic anemia.
Magnesium  deficiency  (choice D)  is  uncommon,  but can  cause  decreased  reflexes, and  blunts  the parathyroid response to hypocalcemia.
Selenium deficiency (choice E) is rare, but may cause a reversible form of cardiomyopathy.

zhangheng1020UID：2130513

Explanation:
The  correct  answer is  C.  Both  pyruvate  kinase  deficiency and  glucose-6-phosphate  dehydrogenase deficiency  are  red  cell  enzyme  deficiencies characterized  clinically  by  long  "normal"  periods interspersed  with  episodes of  hemolytic  anemia  triggered by  infections  and  oxidant drug  injury (antimalarial  drugs, sulfonamides,  nitrofurans). In  both of  these conditions,  the cell  morphology between hemolytic episodes is usually normal or close to normal.
The  α(choice  A) and β(choice  B) thalassemias, in their  major forms, are characterized by persistent severe anemia. In the trait forms, they are charactertized by mild anemia.
Hereditary  spherocytosis  (choice D) is characterized  by intermittent hemolysis, but, unlike pyruvate kinase  deficiency  and glucose-6-phosphate dehydrogenase deficiency,  oxidant drugs are not a specific trigger for hemolysis.
Iron  deficiency anemia  (choice E)  is characterized  by chronic  anemia with  hypochromic, microcytic erythrocytes.

zhangheng1020UID：2130513

To which of the following diseases is pyruvate kinase deficiency most similar clinically?
A. α-thalassemia
B. β-thalassemia
C. Glucose-6-phosphate dehydrogenase deficiency
D. Hereditary spherocytosis
E. Iron deficiency anemia