CompTIA
Checkpoint
ISC
HP
Dell
EC-Council
EXIN
Fortinet
ITIL®
Juniper
PMI
Microsoft
VMware
Avaya
Google
Salesforce
Amazon
Palo Alto Networks
Certiport
ISC2
All Vendors
  2. Home
  3. Exams
  4. USMLE
  5. STEP1

Free STEP1 Exam Braindumps (page: 70)

Page 69 of 213
PDF with 845 Questions

When cells acquire sufficient energy such that the rate of flux through the tricarboxylic acid (TCA) cycle declines, excess acetyl-CoA that cannot be oxidized is predominantly converted into fat. In order for the carbons in mitochondrial acetyl-CoA to serve as a precursor for fat synthesis, they must be delivered to the cytosol. Which of the following represents the molecule used to transport acetyl-CoA to the cytosol?

  1. acetyl-CoA
  2. carnitine
  3. citrate
  4. beta-hydroxybutyrate
  5. pyruvate

Answer(s): C

Explanation:

Acetyl-CoA cannot freely diffuse across the membranes of the mitochondria, nor is there a transport mechanism to move the molecule to the cytosol. This ensures that all acetyl-CoA generated by PDH or fat oxidation will be used for energy production. However, as cellular energy demand falls, carbon atoms can be diverted into storage molecules such as glycogen and fatty acids. To move acetyl-CoA out of the mitochondria to the cytosol it must first be converted to citrate, which can be ransported by the TCA transport system (below figure).

As indicated, acetyl-CoA (choice A) is "trapped" inside the mitochondria. Carnitine (choice B) is necessary for transport of fatty acids into the mitochondria prior to their oxidation, but plays no role in acetyl-CoA transport. Ketone bodies such as beta-hydroxybutyrate (choice D) are generated in hepatic mitochodria when acetyl- CoA is in excess. However, usage of ketone bodies by nonhepatic tissues involves conversion to acetyl-CoA with the mitochondria, not the cytosol. Pyruvate (choice E) can be transported into the mitochondria but not out.



Which of the following is true with respect to the actions of the mineralocorticoids?

  1. decrease carbohydrate metabolism
  2. increase appearance of the secondary sex characteristics
  3. increase synthesis of androgens
  4. regulate aldosterone secretion
  5. regulate sodium retention by the kidneys

Answer(s): E

Explanation:

The principal mineralocorticod, produced by the zona glomerulosa cells of the adrenals, is aldosterone.
Synthesis of aldosterone is primarily controlled by the rennin-angiotensin system and is thus involved in control of blood pressure. Aldosterone causes sodium reabsorption by the kidneys, which in turn regulate water balance, which leads to increases in blood pressure by increasing fluid volume. Aldosterone action does not directly lead to decreased carbohydrate metabolism (choice A), is not a sex characteristic determining steroid hormone (choice B), does not result in increased synthesis of androgens (choice C), and does not regulate its own secretion (choice D).



A 9-month-old child is presented to the emergency room by his parents who report that he has been vomiting and has severe diarrhea. The episodes of vomiting began when the parents started feeding their child cow's milk. The infant exhibits signs of failing to thrive. Laboratory tests show elevated blood galactose, hypergalactosuria, metabolic acidosis, albuminuria, and hyperaminoaciduria. These clinical and laboratory findings are most consistent with which of the following disorders?

  1. alkaptonuria
  2. essential fructosuria
  3. hereditary galactosemia
  4. Menkes disease
  5. von Gierke disease

Answer(s): C

Explanation:

Severe hereditary galactosemia presents in the first few months of life with symptoms that include poor feeding and associated weight loss, vomiting, diarrhea, lethargy, and hypotonia. Clinical findings will include those presented in the case. The symptoms are aggravated by consumption of cow's milk and can be resolved provided proper diagnosis is made and treatment is started early. Alkaptonuria (choice A) results from the accumulation of homogentisic acid, a by-product of tyrosine catabolism, in the urine and tissues.
Oxidation of homogentisate in the urine causes it to turn dark and in the tissues results in ochronosis, which refers to the ochre color of the deposits in connective tissue, bones, and other organs. Essential fructosuria (choice B) is a benign asymptomatic metabolic disorder manifesting with alimentary hyper- fructosemia and fructosuria. Menkes disease (choice D) results from a defect in intracellular copper transport and the symptoms of the disease are caused by loss of function of copper- dependent enzymes.
Symptoms include abnormal (kinky) hair and pigmentation, cerebral degeneration, failure to thrive, and skin laxity. Symptoms of von Gierke disease (choice E) result from the excessive accumulation of glycogen in liver, kidney, and intestinal mucosa. Symptoms include growth retardation, hypoglycemia, heptomegaly, hyperlipidemia, lactic acidemia, and hyperuricemia.



Continued consumption of calories in excess of energy expenditure will eventually lead to obesity, a current major health problem in the United States. A major contributing factor in obesity is a disorder in fuel partitioning, as evidenced by a lower rate of fat oxidation in obese individuals. Which of the following situations would best explain a reduction in overall fat metabolism in these individuals?

  1. An increase in the hepatic ATP/ adenosine diphosphate (ADP) ratio increases incorporation of carbon into fatty acids by causing an inhibition in acetyl-CoA oxidation in the TCA cycle.
  2. Decreased hepatic gluconeogenesis which requires acetyl-CoA from fatty acid oxidation, thus fat oxidation is secondarily inhibited.
  3. Increased levels of malonyl-CoA occur in these individuals leading to inhibition of carnitine palmitoyltransferase I.
  4. Insulin-induced decrease in the activity of acetyl-CoA carboxylase (ACC) causing reduction in fatty acid synthesis.
  5. Insulin-induced repression of fatty acid synthase (FAS) activity.

Answer(s): C

Explanation:

The oxidation of long-chain fatty acids is initiated by the sequential action of carnitine palmitoyltransferase-I (CPTI), which is located in the outer mitochondrial membrane, and carnitine palmitoyltransferase II, which is located in the inner mitochondrial, together with a carnitine- acylcarnitine translocase. Major control over the process is exerted at the level of CPTI by virtue of the unique inhibitability of this enzyme by malonyl- CoA. Thus, CPTI has a pivotal role in lipid metabolism. Obese individuals have higher overall dietary intake of lipid and the disruption in fat metabolism exacerbates the increases in malonyl-CoA, which propagates the fuel partitioning disorder. Although an increase in hepatic ATP (choice A) will lower the flux through the TCA cycle, this does not constitute a mechanism for a disruption in fuel partitioning. The rate of hepaticgluconeogenesis (choice B) does not significantly affect fat metabolism. Obese individualsactually have reduced responses to insulin (choiceD) and thus any insulin-mediated effects on ACC would be minimal.
Insulin does not repress FAS activity (choice E).






Post your Comments and Discuss USMLE STEP1 exam with other Community members:

STEP1 Exam Discussions & Posts