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NBME 30 Exam Questions and Answers
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NBME 30 Exam Questions and Answers1
Exam Section 1: Item 1 of 50
National Board of Medical Examiners®
Comprehensive Basic Science Self-Assessment
1. Shortly after delivery, a full-term male newborn is found to have black hair with a
white forelock. His mother, a brunette, also has a white forelock and wears hearing aids.
Physical examination shows heterochromia of irides. Otoacoustic emissions testing and
brain stem auditory evoked responses show bilateral sensorineural hearing loss. Which
of the following is the most likely cause of the findings in this patient?
A) Abnormal neural crest development
B) Abnormality of connexins
C) Deficiency of homogentisic acid oxidase activity
D) Deficiency of tyrosinase activity
E) Failure of internalization of melanin granules by keratinocytes
F) Failure of melanosome transportation along dendrites - Answers -A.
Abnormal neural crest development leads to Waardenburg syndrome. Waardenburg
syndrome is a syndrome of patchy depigmentation of the skin, hair, irises, and cochlear
dysfunction that primarily illustrates an autosomal dominant inheritance pattern.
Because of
genetic mutations of genes encoding transcription factors, neural crest cells do not
properly differentiate into melanoblasts (melanocyte precursors), or melanoblasts do not
migrate to their appropriate location. Patients typically have a white forelock and
eyelashes,
depigmented skin patches, iridic heterochromia, and sensorineural deafness. The eyes
may also be laterally displaced. The clinical diagnosis may be confirmed with genetic
testing. Treatment includes audiologic evaluation and genetic consultation.
Incorrect Answers: B, C, D, E, and F.
An abnormality of connexins (Choice B) would lead to abnormal formation of the plasma
membrane channels of diverse cell types. Different combinations of sensorineural
hearing loss, ichthyosis, alopecia, and peripheral neuropathy may occur.
Depigmentation
would be atypical.
Deficiency of homogentisic acid oxidase activity (Choice C) would lead to decreased
metabolism of the amino acids phenylalanine and tyrosine, which instead degrade into
homogentisic acid. Homogentisic acid accumulates in the skin and joints, causing
increased
pigmentation and arthritis, respectively. Depigmentation would be atypical.
Deficiency of tyrosinase activity (Choice D) occurs in oculocutaneous albinism, which
presents with uniformly hypopigmented hair and skin (versus the patchy depigmentation
of Waardenburg syndrome) and eye abnormalities (eg, iris hypopigmentation, refractive
,errors, nystagmus). In tyrosinase deficiency, melanocytes are unable to synthesize
melanin from the amino acid tyrosine. Iridic heterochromia and sensorineural deafnes
2
Exam Section 1: Item 2 of 50
National Board of Medical Examiners®
Comprehensive Basic Science Self-Assessment
2. During an experiment, a solution of mixed fatty acids is injected into the duodenum of
an experimental animal. Under these conditions, the clearance rate of an intravenous
glucose load from the circulation is doubled. In contrast, an injection of an equal
volume amount of 0.9% saline into the duodenum has much less effect on the plasma
clearance rate of glucose. These findings are most likely caused by the secretion of
which of the following hormones?
A) Gastrin
B) Glucose-dependent insulinotropic peptide
C) Motilin
D) Secretin
E) Somatostatin - Answers -B.
Glucose-dependent insulinotropic peptide (GIP) is secreted by K cells in the duodenum
and jejunum and functions to decrease gastric acid production and stimulate insulin
release from the pancreas. Its secretion is promoted by the presence of fatty acids,
amino
acids, and intestinal glucose. While serum glucose also stimulates insulin secretion by
the pancreas, the effect of intraluminal glucose on GIP and subsequent insulin secretion
leads to increased concentrations of insulin compared to parenteral glucose
administration. Insulin promotes peripheral tissue uptake of glucose, glycolysis,
glycogen synthesis, protein synthesis, and fatty acid synthesis, resulting in decreased
glucose concentration in the serum.
Incorrect Answers: A, C, D, and E.
Gastrin (Choice A) is produced by G cells in the gastric antrum and stimulates parietal
cells within the gastric body to produce hydrochloric acid. Gastric acid has no effect on
serum glucose concentration.
Motilin (Choice C) is secreted by the small intestine and stimulates intestinal peristalsis.
Motilin receptors are targeted by erythromycin and metoclopramide, used
therapeutically in gastroparesis.
Secretin (Choice D) is produced by duodenal S cells. It promotes the release of
bicarbonate-rich pancreatic secretions and bile and inhibits gastric acid production.
Somatostatin (Choice E) is a regulatory peptide secreted by D cells of the pancreas and
gastrointestinal mucosa that inhibits gastric acid and pepsinogen secretion, gallbladder
contraction, and insulin and glucagon release. Somatostatin would have an indirect
effect on glucose through counterregulatory action of both insulin and glucagon.
Educational Objective: Glucose-dependent insulinotropic peptide (GIP) is secreted by K
cells in the duodenum and jejunum, and it functions to decrease gastric acid production
and
3
,Exam Section 1: Item 3 of 50
National Board of Medical Examiners®
Comprehensive Basic Science Self-Assessment
3. A 53-year-old man comes to the physician because of a 6-day history of shortness of
breath, cough, and pleuritic chest pain. His temperature is 38.9°C (102°F), and
respirations are 35/min. Sputum is purulent and rust colored. Physical examination
shows
decreased breath sounds, and crackles are heard at the left base. A Gram stain of
sputum shows gram-positive diplococci. A chest x-ray shows left-sided lobar
consolidation. Which of the following anatomic structures most likely allowed rapid
spread of
organisms between alveoli to involve the entire left lobe in this patient?
A) Alveolar capillaries
B) Germinal centers
C) Lymphatic capillaries
D) Pores of Kohn
E) Vascular sinusoids - Answers -D.
The pores of Kohn form connections between alveoli and are present in normal lung
tissue. They are composed, at least in part, of type Il alveolar cells and allow for the
passage of air, fluid, phagocytes, and in the setting of pneumonia, bacteria between
adjacent
alveoli. By allowing equilibration between adjacent alveoli, the pores of Kohn aid in
normal oxygenation and in the prevention of atelectasis. However, infectious organisms
and inflammation may also spread between adjacent alveoli through these apertures.
Incorrect Answers: A, B, C, and E.
Alveolar capillaries (Choice A) surround each alveolus and are crucial for gas
exchange. However, they do not provide a direct connection between adjacent alveoli,
provided that the alveolar and capillary endothelia remain intact.
Germinal centers (Choice B) allow for the development of B lymphocytes and are
important in mounting an adaptive immune response to pulmonary pathogens. They do
not form connections between alveoli.
Lymphatic capillaries (Choice C) allow for the drainage of interstitial fluid from the lung
parenchyma to the lymphatic ducts and eventually to the heart. These passages do not
form direct anastomoses between adjacent alveoli.
Vascular sinusoids (Choice E) are present in hepatic tissue and create anastomoses
between the portal and systemic circulation, allowing for the exchange of nutrients with
hepatocytes. They are not present in pulmonary tissue.
Educational Objective: The pores of Kohn form connections between alveoli and are
present in normal lung tissue. They allow for the passage of air, fluid, phagocytes, and,
in the setting of pneumonia, bacteria between adjacent alveoli.
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Exam Section 1: Item 4 of 50
National Board of Medical Examiners®
Comprehensive Basic Science Self-Assessment
4. A female newborn is born at 39 weeks' gestation following an uneventful pregnancy.
The Apgar scores are 10 at 1 and 5 minutes, respectively. The newborn is at the 50th
percentile for length and 25th percentile for weight. She has normal morphologic
features. The parents are told she has an enzyme defect that, if left untreated, will result
in failure to attain early developmental milestones, microcephaly, hyperactivity,
seizures, intellectual disability, and a mousy odor of the skin and urine. The patient's
diet
needs to contain which of the following to provide substrate for catecholamine
production?
A) Dihydroxyphenylalanine
B) Dopamine
C) Epinephrine
D) Norepinephrine
E) Phenylalanine
F) Tyrosine - Answers -F.
Tyrosine supplements are required for catecholamine production in this patient with
phenylketonuria (PKU). PKU results from a deficiency in the enzyme phenylalanine
hydroxylase (PAH) that converts phenylalanine to tyrosine. Deficiency results in
accumulation of
phenylalanine and its metabolites phenylacetate and phenyllactate. Rarely, mutations in
enzymes that either produce or recycle tetrahydrobiopterin (BH , which is a cofactor for
PAH, can have similar clinical manifestations as classic PKÚ. Phenylalanine is a
precursor to tyrosine, and tyrosine is used to synthesize catecholamines such as
dopamine, epinephrine, and norepinephrine. Tyrosine is converted to
dihydroxyphenylalanine by tyrosine hydroxylase, followed by conversion to dopamine by
DOPA-decarboxylase.
Dopamine can subsequently be converted to norepinephrine, and methylation of
norepinephrine results in formation of epinephrine. Patients with PKU from PAH
deficiency require supplementation with tyrosine to bypass the dysfunctional enzymatic
step of
converting phenylalanine to tyrosine.
Incorrect Answers: A, B, C, D, and E.
Dihydroxyphenylalanine (Choice A), abbreviated as DOPA, is a precursor to dopamine
and is formed via the action of tyrosine hydroxylase. This enzymatic pathway is
functional in patients with PKU so supplementation with DOPA is not required. Similarly,
dopamine (Choice B) is created from DOPA by the enzyme DOPA decarboxylase, while
epinephrine (Choice C) and norepinephrine (Choice D) are made from dopamine by
separate enzymes, none of which are deficient in PKU. Supplementation of tyrosine is
sufficient to allow for normal synthesis of all of these catecholamines.
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