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: 71)

Page 70 of 213
PDF with 845 Questions

A3-month-old infant who otherwise appeared normal during the first 2 months of life except for a bout of hyperbilirubinemia is now clearly exhibiting developmental delay. In addition, the infant's hair has become grayish and dull and there is a stubble of broken hair over the occiput and temporal regions. The facial appearance has also changed such that the infant has very pudgy cheeks, abnormal eyebrows, and sagging jowls. The occurrence of frequent convulsions was the stimulus for the parents to bring their child to the emergency room. These rapidly deteriorating symptoms are indicative of which of the following disorders?

  1. Crigler-Najjar syndrome type I
  2. Gilbert syndrome
  3. hemochromatosis
  4. Menkes disease
  5. Refsum disease

Answer(s): D

Explanation:

Menkes disease is an X-linked recessive disorder that is manifest by a defect in copper absorption. This defect leads to dysfunction of numerous enzymes that need copper as a cofactor, leading to the typical symptoms observed in this patient. In fact, Menkes disease is also referred to as steely hair disease, because of the characteristic brittleness of the hair, which is easily broken. Crigler-Najjar syndrome type I (choice A) is also due to defective bilirubin metabolism as a result of a loss of UDP- glucuronosyltransferase (UGT) activity. UGT is required to transfer 2 moles of glucuronic acid to bilirubin, generating bilirubindiglucuronide, which makes bilirubin much more water soluble and therefore facilitates its excretion. Crigler-Najjar syndrome results in nonhemolytic icterus (jaundice) within the first few days of life and is generally fatal during neonatal life due to severe kernicterus. Gilbert syndrome (choice B) results from a defect in bilirubin metabolism. It is typically diagnosed in young adults and is characterized by mild, chronic, and unconjugated hyperbilirubinemia without associated hemolysis. Hemochromatosis (choice C) is the term applied when organ structure and function are impaired by the presence of excess amounts of iron. The liver, heart, pancreas, skin, joints, and endocrine organs are the principal tissues affected by iron accumulation. Symptoms include cirrhosis, cardiomyopathy, arthritis, abnormal skin pigmentation, and hypogonadism, as well as diabetes mellitus. Refsum disease (choice E) results from a defect in the metabolism of phytanic acid, a plant lipid which must be oxidized by a separate pathway from that of animal fats. Cardinal symptoms include retinitis pigmentosa, peripheral neuropathy, and cerebellar ataxia.



Presentation of the platelet membrane protein complex, GPIIb-GPIIIa, which binds with von Willebrand factor, is necessary for which of the following?

  1. activation of PKC leading to phosphorylation of myosin light chain and platelet morphology changes
  2. cleavage and activation of high molecular weight kininogen
  3. inducing platelet crosslinking
  4. release of thrombin from platelet granules
  5. stimulation of endothelial and smooth muscle cell interaction resulting in vasoconstriction

Answer(s): C

Explanation:

The platelet membrane glycoprotein complex, GPIIb-GPIIIa, is a receptor for fibrinogen. Presentation of GPIIb-GPIIIa on the surface of activated platelets leads to platelet crosslinking. The cross-linking occurs between activated platelets that are adhered to the endothelial cell surface via interaction of the platelet surface complex GPIa-GPIIa with exposed collagen, as well as between platelets in the local circulation.
The effect is to ensure that a loose platelet plug forms rapidly at the site of vascular injury stemming the flow of blood until a fibrin clot can be formed. Activation of platelet PKC (choice A) is the consequence of thrombin binding to platelet thrombin receptors. Cleavage of high molecular weight kininogen (choice B) occurs upon the activation of factor XII. Release of thrombin from platelet alpha granules (choice D) occurs in response to platelet activation. Activation of platelets must first occur in order for GPIIb- GPIIa to be presented on the surface and, thus, this glycoprotein complex is not directly involved in platelet activation and granule release. Vasoconstriction, in concert with hemostasis (choice E), occurs primarily as a result of the action of serotonin released from alpha granules of activated platelets.



Synthesis of glycogen is inhibited in hepatocytes in response to glucagon stimulation primarily as a result of which of the following?

  1. a decrease in the level of phosphoprotein phosphatase
  2. a decrease in the level of phosphorylated phosphorylase kinase
  3. a decrease in the levels of phosphorylated phosphoprotein phosphatase inhibitor-1
  4. an increase in the level of the dephosphorylated form of glycogen synthase
  5. an increase in the level of the phosphorylated form of glycogen synthase

Answer(s): E

Explanation:

Glucagon is released from the pancreas in response to low blood glucose and stimulates hepatocytes to synthesize glucose for delivery to the blood. Therefore, it would be counterproductive for hepatocytes to divert any of the gluconeogenically derived glucose into glycogen. This is accomplished by inhibition of glycogen synthase. Glucagon exerts its effects on the liver through the glucagon receptor. When glucagon binds, the receptor activates adenylate cyclase leading to increased production of cAMP. In turn, cAMP activates cAMP-dependent protein kinase, which then phosphorylates a number of substrates. Glucagon has no effect on the level of phosphoprotein phosphatase (choice A). One of the substrates of PKA is glycogen synthase/phosphorylase kinase. Therefore, there would not be a decrease in the level of phosphorylated phosphorylase kinase (choice B). In turn, synthase/phosphorylase kinase phosphorylates glycogen phosphorylase and glycogen synthase. Therefore, there is no increase in the level of dephosphorylated glycogen synthase (choice D). Phosphorylation inhibits glycogen synthase activity and activates phosphorylase. In addition, PKA itself can phosphorylate glycogen synthase. The net effect is an increase in the rate of glucose phosphorolysis from glycogen and a reduced incorporation of glucose into glycogen. An additional PKA substrate is phosphoprotein phosphatase inhibitor-1, and therefore therewould not be a decrease in the level of the phosphorylated form of this enzyme (choice C).



Which of the following occurs in the lipidosis known as Tay-Sachs disease?

  1. Ganglioside GM2 is not catabolized by lysosomal enzymes.
  2. Phosphoglycerides accumulate in the brain.
  3. Synthesis of a specific ganglioside is decreased.
  4. Synthesis of a specific ganglioside is excessive.
  5. Xanthomas, due to cholesterol deposition, are observed.

Answer(s): A

Explanation:

In the genetic disorder known as Tay-Sachs disease, ganglioside is not catabolized. As a
consequence, the ganglioside concentration is elevated many times higher than normal. The functionally absent lysosomal enzyme is beta- N-acetylhexosaminidase. The elevated results in
irreversible brain damage to infants, who usually die before the age of 3 years. Under normal conditions, this enzyme cleaves N-acetylgalactosamine from the oligosaccharide chain of this complex sphingolipid, allowing further catabolism to occur. The cause of most lipidoses (lipid storage diseases) is similar. That is, a defect in catabolism of gangliosides causes abnormal accumulation. None of the other choices (B, C, D, and E) result in glycosphinogolipidoses such as is characteristic of Tay-Sachs disease.






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

STEP1 Exam Discussions & Posts