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 correct
answers >> A.
Abnormal neural crest development leads to Waardenburg syndrome.
Waardenburg syndrome is a syndrome of patchy depigmentation of the
GRADED A+
,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
GRADED A+
,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 correct 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
GRADED A+
, 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
GRADED A+
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 correct
answers >> A.
Abnormal neural crest development leads to Waardenburg syndrome.
Waardenburg syndrome is a syndrome of patchy depigmentation of the
GRADED A+
,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
GRADED A+
,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 correct 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
GRADED A+
, 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
GRADED A+
