BIO chem enzymes step1 contd

Question 1

The mutation responsible for this patient’s β-thalassemia most likely leads to defects during which of the following processes?

Answer 1

Posttranscriptional modification.

A single-nucleotide substitution in a noncoding intervening region (or intron) may cause aberrant mRNA splicing, resulting in no protein synthesis or synthesis of a nonfunctional gene product.

Question 2

A young patient presents with severe cramps after physical exertion followed by passage of “red tinged” urine. She has no other symptoms. These findings are suggestive of?

Answer 2

McArdle disease. Onset of the disease typically occurs in adolescence or early adulthood and is characterized by muscle cramping, rapid fatigue, and poor endurance during exertion.
McArdle disease is a glycogen storage disorder (see table) in which myophosphorylase (an isozyme of glycogen phosphorylase) is deficient in muscle.

Question 3

α1,6-Glucosidase is the enzyme responsible for ?

Answer 3

the debranching of glycogen. It is implicated in Cori disease in which muscle cramping is not a typical feature.

Cystathionine synthase is the deficient isoenzyme in patients with homocystinuria, who are at risk for atherosclerosis and vascular disease but do not typically experience muscle cramping.

Question 4

Glucose-6-phosphatase deficiency causes ?

Answer 4

von Gierke disease, characterized by a severe fasting hypoglycemia, increased glycogen in the liver, hepatomegaly, and increased blood lactate.

Question 5

Lysosomal α1,4-glucosidase is the defective enzyme in patients with?

Answer 5

Pompe disease, which presents with respiratory difficulties, cardiomegaly, and progressive loss of muscle tone, leading to death early in life.

Question 6

The breathlessness, peripheral edema, and fatigue in this child are signs of a heart condition. In addition, the findings of hepatomegaly, muscular hypotonia, and decreased acid maltase levels, all point toward ?

Answer 6

a glycogen storage disease. Pompe disease is a type II glycogen storage disease that primarily affects the heart (“Pompe trashes the Pump”).

Question 7

In Pompe disease, lysosomal α1,4-glucosidase is absent, which is necessary for the hydrolysis of the outer branches of glycogen. As a result, glycogen is deposited in the myocardium. By the sixth month of life, children with Pompe disease experience?

Answer 7

developmental delays, feeding problems, and eventual heart failure. Skeletal muscle and the liver are also affected. ECG shows short PR intervals with large QRS complexes signaling biventricular hypertrophy. Cardiomegaly is also evident on x-ray of the chest.

Question 8

α-Galactosidase deficiency would present with ?

Answer 8

peripheral neuropathy symptoms (Fabry disease). β-Glucocerebrosidase deficiency (Gaucher disease) is characterized by femur necrosis and bone crisis.

Question 9

Patients with glucose-6-phosphatase deficiency have signs and symptoms of?

Answer 9

severe hypoglycemia and increased blood lactate levels. Glycogen phosphorylase deficiency presents with severe muscle cramping.

Question 10

A 7-day-old infant is brought to his pediatrician because of lethargy and feeding problems. He is admitted to the hospital for failure to thrive. Shortly after admission the infant has a seizure. A biochemistry evaluation reveals a deficiency in α-ketoacid dehydrogenase.

To prevent intellectual disability and death, intake of which of the following amino acids should be restricted in this patient?

Answer 10

Leucine

Maple syrup urine disease (MSUD) is caused by a defect in the branched-chain α-keto acid dehydrogenase complex. This enzyme is responsible for the catabolism of the branched-chain amino acids, leucine, isoleucine, and valine, illustrated in the diagram. Therefore, restriction of dietary intake of these three amino acids is the treatment for MSUD.

Question 11

Tryptophan is the precursor substrate for serotonin and niacin. The first enzyme involved in tryptophan catabolism is?

Answer 11

tryptophan oxygenase. Deficiency in this enzyme, or other enzymes in the catabolism of tryptophan, will lead to lower levels of those substances. Since it is not a branched-chain amino acid it does not get catabolized by the α-keto acid dehydrogenase complex.

Question 12

Tyrosine is a precursor to many different compounds in the human body including L-DOPA. The first enzyme involved in tyrosine catabolism is?

Answer 12

tyrosine transaminase. It is not catabolized by the α-keto acid dehydrogenase complex and does not cause maple syrup urine disease.

Question 13

Threonine does not get catabolized by α-keto acid dehydrogenase complex and does not cause maple syrup urine disease. It gets converted into?

Answer 13

pyruvate by threonine dehydrogenase.

Question 14

Alanine is not a branched-chain amino acid. It is not catabolized by α-keto acid dehydrogenase complex and does not cause?

Answer 14

maple syrup urine disease. There is no known metabolic defect of alanine catabolism.

Question 15

A patient with a history of rhinitis and asthma presents to the emergency department with chest tightness after having taken aspirin. This patient has most likely developed?

Answer 15

aspirin-exacerbated respiratory disease (AERD) as a result of her aspirin ingestion. Reactions typically develop within 3 hours of ingestion and often involve: (1) nasal and ocular symptoms and (2) asthmatic symptoms. In our patient, these manifest as rhinorrhea, infected conjunctiva, and wheezes.

Question 16

Aspirin and other nonsteroidal anti-inflammatory drugs inhibit the cyclooxygenase pathway. Patients with baseline dysregulation of arachidonic acid metabolism can experience severe asthmatic and nasoocular symptoms after ingestion of a COX-inhibitor due to ?

Answer 16

increased production of proinflammatory leukotrienes, such as LTC4.

Question 17

Based on the patient’s complaints of muscle weakness in adolescence and calf muscle hypertrophy with fiber degeneration, the diagnosis is most likely?

Answer 17

Becker muscular dystrophy. Becker muscular dystrophy is an X-linked recessive disorder caused by a truncated dystrophin protein, resulting in muscular weakness and replacement of muscle with fatty tissue. The distribution of weakness can be similar to that of Duchenne muscular dystrophy (DMD). However, DMD presents in a younger age group, and its symptoms are more severe and progress much faster than those of Becker muscular dystrophy.

Question 18

This patient with a history of alcohol abuse demonstrrates with a protuberant abdomen, slurred speech, and confusion, which are consistent with a diagnosis of?

Answer 18

decompensated liver failure and hepatic encephalopathy. His protuberant abdomen with a fluid wave, spider angiomata, and alcohol history all point toward portal hypertension caused by alcoholic cirrhosis.

Question 19

Ammonia levels increase in decompensated liver failure and can lead to hepatic encephalopathy. Normally, ammonia is converted to the less-toxic urea in healthy liver cells. The initial step of the urea cycle, the synthesis of carbamoyl phosphate, takes place in?

Answer 19

hepatocyte mitochondria.

Question 20

This patient presents with low HDL levels, and was prescribed ?

Answer 20

Niacin, or vitamin B3, is a therapy for raising HDL levels and, to a lesser extent, for lowering LDL and triglyceride levels. It has fallen out of favor recently, as its effect on cholesterol levels are relatively minimal, and its side effects can be unpleasant.

Question 21

Cracking of the lips and corneal vascularization is often a sign of?

Answer 21

a B2 deficiency.

Dilated cardiomyopathy and edema can be found in patients suffering from wet beriberi, due to a vitamin B1 deficiency.

Question 22

A deficiency in vitamin A can lead to night blindness, dry skin, and immune impairment. Perifollicular and subperiosteal hemorrhages is a sign of ?

Answer 22

vitamin C deficiency.

Question 23

Patients taking niacin describe transient cutaneous flushing episodes, which can be alleviated by taking aspirin. Severe deficiency of niacin, in contrast, results in?

Answer 23

pellagra. The symptoms of pellagra include dermatitis, diarrhea, glossitis (shown in the image), mental confusion, and eventually dementia.

Question 24

In this patient with chronic arthritis and urine that turns dark after several hours, the most likely diagnosis is?

Answer 24

alkaptonuria. Alkaptonuria is an autosomal recessive disease caused by a deficiency of homogentisic acid oxidase, an enzyme in the tyrosine degradation pathway that converts homogentisic acid to maleylacetoacetic acid. In this disease, homogentisic acid builds up in the tissue and is excreted in the urine, and exposure to alkali or oxygen results in the characteristic change of the urine color to dark black.

Question 25

Branched amino acids (valine, leucine, and isoleucine) build up in the serum and urine of patients with?

Answer 25

maple-syrup urine disease, which presents at a young age with vomiting, poor feeding, and urine that smells like maple syrup or burnt sugar. Cystinuria is associated with cystine kidney stone formation.

Question 26

Homocysteine is elevated in homocystinuria, which is characterized by intellectual disability, osteoporosis, marfanoid habitus, kyphosis, downward and inward lens subluxation, thrombosis, and atherosclerosis (stroke and MI).
Levels of phenylalanine are increased in ?

Answer 26

phenylketonuria. Left untreated, patients with this condition present with intellectual disability, growth retardation, seizures, fair skin and hair, eczema, and a musty body odor.

Question 27

This patient, who has an exacerbation of chronic obstructive pulmonary disease (COPD), is receiving a macrolide antibiotic (azithromycin), a ß-agonist (albuterol), an anticholinergic agent (ipratropium), and a corticosteroid (prednisone). His lab test results reveal hyperglycemia (normal random glucose level should be <200 mg/dL), which is likely related to the use of albuterol. Steroids cause?

Answer 27

hyperglycemia by promoting gluconeogenesis. The question can then be simplified to the identification of which reaction occurs in gluconeogenesis.

Question 28

Of the reactions listed, the conversion of oxaloacetate to phosphoenolpyruvate is the only one involved in gluconeogenesis . The conversion of oxaloacetate to phosphoenolpyruvate is catalyzed by ?

Answer 28

phosphoenolpyruvate (PEP) carboxykinase, which functions as a regulatory enzyme in the gluconeogenesis pathway. PEP carboxykinase requires GTP and is an irreversible enzyme in gluconeogenesis.

Question 29

Pyruvate dehydrogenase converts pyruvate to acetyl coenzyme A and connects glycolysis to the citric acid cycle. Malate dehydrogenase converts malate to oxaloacetate in the citric acid cycle. Hexokinase and glucokinase convert?

Answer 29

glucose to glucose-6-phosphate in glycolysis. Similarly, pyruvate kinase is another enzyme in glycolysis; it converts phosphoenolpyruvate to pyruvate. Glycolysis and the citric acid cycle are involved in glucose catabolism rather than glucose production and would not be promoted by steroid administration.

Question 30

teroids promote hyperglycemia by stimulating gluconeogenesis. PEP carboxykinase is a cytosolic enzyme that catalyzes an irreversible step in gluconeogenesis, converting?

Answer 30

oxaloacetate to phosphoenolpyruvate.