biomedical sub test[kalplan 2001] Discussion is welcome!

zhangheng1020UID：2130513

Explanation:
         The correct answer is A. Long-term starvation induces many biochemical changes. Much of the body's energy requirements are normally supplied by serum glucose, but in starvation are supplied by both glucose and lipid-derived ketone bodies, including acetoacetic acid and beta-hydroxybutyric acid. Glucose cannot be synthesized from lipids, and is instead made from amino acids such as alanine in the process of  gluconeogenesis.
         Serum alanine (choice B) drops dramatically in starvation, due to its conversion to glucose.
         Bicarbonate (choice C) levels drop as the bicarbonate buffers the hydrogen ions produced by the ketone bodies.
         Chylomicrons (choice D) are the lipid form seen after absorption of dietary fat, and would drop because the person is not feeding.
         Glucose (choice E) is maintained in the blood at a much lower than normal level during starvation.

zhangheng1020UID：2130513

A 15-year-old girl is seen by a dermatologist for removal of multiple squamous cell carcinomas of the skin. The patient has nearly white hair, pink irises, very pale skin, and a history of burning easily when exposed to the sun.This patient's condition is caused by a disorder involving which of the following substances?
                A. Aromatic amino acids
                B. Branched chain amino acids
                C. Glycolipids
                D. Glycoproteins
                E. Sulfur-containing amino acids

zhangheng1020UID：2130513

Explanation:
         The correct answer is A. The disease is albinism. The most common form of albinism is caused by a deficiency of copper-dependent tyrosinase (tyrosine hydroxylase), blocking the production of melanin from the aromatic amino acid tyrosine. Affected individuals lack melanin pigment in skin, hair, and eyes, and are prone to develop sun-induced skin cancers, including both squamous cell carcinomas and melanomas. Maple syrup urine disease is an example of a disorder of branched chain amino acids (choice B) causing motor abnormalities and seizures.
         Tay-Sachs disease is an example of a disorder of glycolipids (choice C). In this disorder, a deficiency of hexosaminidase A leads to accumulation of ganglioside GM2.
         Hunter's disease is an example of a disorder of glycoproteins (choice D). This mucopolysaccharidosis is inherited as an autosomal recessive trait.
         Homocystinuria disease is an example of a disorder of sulfur-containing amino acids (choice E).

zhangheng1020UID：2130513

A 7-year-old boy is referred to a specialty clinic because of digestive problems. He often experiences severe abdominal cramps after eating a high fat meal. He is worked up and diagnosed with a genetic defect resulting in a deficiency of lipoprotein lipase. Which of the following substances would most likely be elevated in this patient's plasma following a fatty meal?
                 A. Albumin-bound free fatty acids
                 B. Chylomicrons
                 C. HDL
                 D. LDL
                 E. Unesterified fatty acids

zhangheng1020UID：2130513

Explanation:
         The correct answer is B. After eating a high fat meal, triglycerides are processed by the intestinal mucosal cells. They are assembled in chylomicrons and eventually sent into the circulation for delivery to adipocytes and other cells. Chylomicrons are too large to enter cells, but are degraded while in the circulation by lipoprotein lipase. A defect in this enzyme would result in the accumulation of chylomicrons in the plasma.
         Albumin-bound free fatty acids (choice A) is incorrect because fatty acids leave the intestine esterified as triglycerides in chylomicrons.
         HDL (choice C) is not a carrier of dietary fat from the intestine.
         LDL (choice D) would be not be elevated in this patient after a high fat meal. However, VLDL would be elevated if the patient ate a high carbohydrate meal. In this situation, the carbohydrate would be converted into fat in the liver and sent out into circulation as VLDL. VLDL would be unable to be degraded to LDL and, therefore, would accumulate.
         A defect in lipoprotein lipase would cause a decrease, not an elevation of unesterified fatty acids (choice E), since the chylomicrons contain esterified fatty acids.

zhangheng1020UID：2130513

A 38-year-old man in a rural area presents to a physician for an employment physical. Ocular examination reveals small opaque rings on the lower edge of the iris in the anterior chamber of the eye. Nodular lesions are found on his Achilles tendon. Successful therapy should be aimed at increasing which of the following gene products in hepatocyte cell membranes?
                A. Apo B-100
                B. Apo B-100 receptor
                C. Apo E
                D. Apo E receptor
                E. Lecithin cholesterol acyltransferase

zhangheng1020UID：2130513

Explanation:
         The correct answer is B. This man has characteristic signs of familial hypercholesterolemia, an autosomal dominant disorder affecting about 1 in 500 persons. The xanthomas on the Achilles tendon and the arcus lipoides (the opaque rings in the eye) are pathognomonic. Affected individuals have very high LDL cholesterol because of deficient endocytosis of LDL particles by LDL receptors. These receptors recognize the apo B-100  protein cotransported with cholesterol esters in LDL. Treatments aim at increasing genetic expression of LDL receptors (i.e., apo B-100 receptors) to enhance clearance of LDL particles. Dietary changes, a resin drug,  niacin, or an HMG-CoA reductase inhibitor could be tried.
         Apo B-100 (choice A) is the apoprotein of liver-produced lipoproteins such as VLDL, IDL and LDL. It is therefore not in the hepatic cell membranes, and it might be expected to decrease with decreasing concentrations of circulating LDL.
         Apo E (choice C) is an apoprotein found on VLDL, IDL, and chylomicrons, allowing "scavenging" by the liver of remnants or of the lipoprotein itself. It is not found in the hepatocyte membrane.
         Apo E receptor (choice D) would actually be increased by the treatment with hypocholesterolemic agents. However, the apo E receptor is not involved in the scavenging of LDL particles.
         Lecithin cholesterol acyl transferase (choice E) or LCAT, is activated by apo AI, and esterifies free cholesterol in plasma. Plasma levels of HDL cholesterol and apo AI are inversely related to the risk of coronary heart disease.

zhangheng1020UID：2130513

Which of the following structures is common to all sphingolipids?
                 A. Carnitine
                 B. Ceramide
                 C. Diacylglycerol
                 D. Sphingomyelin
                 E. Squalene

zhangheng1020UID：2130513

Explanation:
         The correct answer is B. Sphingolipids are a class of lipids that are structural components of membranes.  Ceramide is a component of sphingolipids. Ceramide is composed of sphingosine, a long-chain amino alcohol with a saturated fatty acid linked to the amino group. Sphingolipids can be differentiated on the basis of the "X" group that is esterified to the terminal hydroxyl group of ceramide.
         Carnitine (choice A) is involved in the oxidation of fatty acids. Carnitine is important in transferring fatty acids  from the cytoplasm into the mitochondria (the carnitine shuttle).
         Diacylglycerol (choice C) is the alcohol common to all phospholipids. The second alcohol (e.g., choline, ethanolamine, serine) contributes the polar head that distinguishes the different classes of phospholipids. Like  sphingolipids, phospholipids are found in membranes.
         Sphingomyelin (choice D) is a sphingolipid with phosphocholine as its "X" group. It is a component of the myelin sheath.
         Squalene (choice E) is a 30-carbon intermediate in the synthesis of cholesterol.

zhangheng1020UID：2130513

A 47-year-old male patient presents with painful arthritis in the right big toe and uric acid renal stones. He has been taking allopurinol for his condition. What biochemical defect would likely be found in this patient?
                A. A defect in urea synthesis
                B. An abnormality of the purine degradation pathway
                C. An inability to synthesize non-essential amino acids
                D. Defective topoisomerases
                E. Increased levels of leukotrienes

zhangheng1020UID：2130513

Explanation:
         The correct answer is B. This patient has gout, characterized by painful joints due to the precipitation of uric  acid crystals caused by excessive production of uric acid (a minority of cases are associated with  underexcretion of uric acid). Kidney disease is also seen due to accumulation of uric acid in the tubules. The  disease mostly affects males, and is frequently treated with allopurinol, an inhibitor of xanthine oxidase.  Xanthine oxidase catalyzes the sequential oxidation of hypoxanthine to xanthine to uric acid.
         A defect in urea synthesis (choice A) would result in the accumulation of ammonia.
         Phenylketonuria is a disease in which tyrosine cannot be produced from phenylalanine (choice C). It is characterized by a musty body odor and mental retardation.
         Defective topoisomerases (choice D) would affect DNA unwinding, and therefore replication.
         Leukotrienes (choice E) are potent constrictors of smooth muscle and would more likely lead to  bronchoconstriction.

zhangheng1020UID：2130513

A newborn vomits after each feeding of milk-based formula, and does not gain weight. Biochemical testing reveals a severe deficiency of galactose-1-phosphate uridyltransferase, consistent with homozygosity. If this condition goes untreated, what is the likely outcome for this patient?
                A. Benign disease except for cataract formation
                B. Chronic emphysema appearing in early adulthood
                C. Chronic renal failure appearing in adolescence
                D. Death in infancy
                E. Gastrointestinal symptoms that remit with puberty

zhangheng1020UID：2130513

Explanation:
         The correct answer is D. Galactosemia occurs in two very different clinical forms. Deficiency of galactokinase produces very mild disease with the only significant complication being cataract formation. In contrast, homozygous deficiency of galactose-1-phosphate uridyltransferase produces severe disease culminating in death in infancy. In addition to galactosemia and galactosuria, these patients have impaired renal tubular resorption leading to aminoaciduria, gastrointestinal symptoms, hepatosplenomegaly, cataracts, bleeding diathesis, hypoglycemia, and mental retardation. Pathologically, the CNS shows neuronal loss and gliosis and the liver shows fatty change progressing to cirrhosis.
         Benign disease with cataract formation (choice A) is characteristic of galactokinase deficiency.
         Chronic emphysema (choice B) is not associated with homozygous galactose-1-phosphate uridyltransferase deficiency, but rather with alpha 1-antitrypsin deficiency.
         Impaired tubular reabsorption (producing aminoaciduria) is seen within a few days or weeks of feeding milk to an infant with severe galactosemia, as opposed to chronic renal failure appearing in adolescence (choice C).
         Gastrointestinal symptoms (choice E) certainly occur in homozygous galactose-1-phosphate uridyltransferase deficiency, but they would not be expected to remit with puberty. Instead, most untreated infants with this disorder show failure to thrive and die in infancy from wasting and inanition.

zhangheng1020UID：2130513

A 20-year old female who is 2 months pregnant remembers that she had phenylketonuria (PKU) as a child and required a special diet. Tests confirm markedly elevated maternal serum levels of phenylalanine and phenylacetic acid. Genetic studies have not been performed on the father. What should the physician tell the parents regarding the welfare of the child?
                A. Childhood phenylalanine restriction is sufficient to protect the health of her child.
                B. Further information is required to ascertain if the fetus is at risk.
                C. The fetus is at no health risk if it is heterozygous for the PKU gene.
                D. The fetus is at no health risk if phenylalanine levels are normalized by the third trimester.
                E. The mother's hyperphenylalaninemia may have already harmed the fetus.

zhangheng1020UID：2130513

Explanation:
         The correct answer is E. Phenylalanine crosses the placenta and, if maternal serum levels are elevated, acts as a teratogen to the developing fetus. This condition is known as maternal PKU. Although the mother can fare well with substantial elevations in serum phenylalanine concentration, the children born to such women are usually profoundly retarded and may have multiple birth defects.
         Although dietary modifications (choice A) can prevent the neurological and dermatologic manifestations of PKU  in a child, the fetus is still at risk from maternal PKU.
         Further information regarding the cause of this woman's hyperphenylalaninemia (choice B) is not needed, since  the fetus is exposed to teratogenic levels of phenylalanine.
         Children born to mothers with untreated PKU develop maternal PKU even if they are heterozygous for the PKU  gene (choice C). Fetal phenylalanine hydroxylase cannot compensate for the high maternal levels of  phenylalanine.
         The critical period in development during which teratogenic materials affect the growing organs is between the  3rd and 8th weeks of gestation. By the end of the 2nd month (compare with choice D), the damage caused by the maternal PKU has already occurred.