DANIL HAMMOUDI.MD
USMLE REVIEW FOR STEP1
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Initiation of
the action potential (phase 0) in ventricular myocytes is due to activation of the rapid Na+
channels The diagram depicts a typical ventricular myocyte action potential. The stable resting membrane
potential (phase 4) is approximately -90 mV. This highly polarized state
indicates that the membrane has a high conductance (permeability) to K+.
The rapid reversal of the membrane potential in ventricular myocytes (phase 0) is due to the opening of the rapid
Na+ channels and the approach of the membrane to the equilibrium potential
for Na+ (+60 mV). During phase 1, the membrane potential begins to repolarize. This is caused by the transient opening of
K+ channels, as well as the inactivation of the Na+ current. Phase 2 (the
plateau phase) results from the opening of the slow Ca++ channels. The
influx of Ca++ is balanced by the efflux of K+, so a stable plateau
potential is maintained. During phase 3, the membranes' conductance to K+
increases and the membrane repolarizes. Myocytes do not possess automaticity and depend on the spread of electrical
activity for activation. |
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A patient with eczema, thrombocytopenia, and recurrent infections is best characterized by which disorder? Deficiency of adenosine deaminase The Wiskott-Aldrich syndrome is a sex-linked
recessive disease with a characteristic triad of thrombocytopenia, eczema,
and recurrent infections. Additional clinical findings include bloody diarrhea, cerebral
hemorrhage, septicemia, severe infections by encapsulated bacteria, and an
increased propensity for malignant lymphomas. Laboratory findings include low lymphocyte counts as the T cells
decrease with subsequent impairment in cellular immunity, low
immunoglobulin M levels, and increased levels of immunoglobulin G,
immunoglobulin A, and particularly immunoglobulin E. |
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The triad of Howell-Jolly bodies, target cells, and reticulocytosis is most compatible with which clinical finding? Splenectomy The finding of Howell-Jolly bodies, target cells, and reticulocytosis is most compatible with splenectomy. The spleen has many functions, including the removal of defective and aged blood cells, hematopoiesis (in times of severe stress when the marrow is unable to meet the needs of the organism), iron conservation, blood cell storage, and host defense in the form of humoral immunity. Therefore, splenectomy can result in a number of hematologic abnormalities. Following splenectomy, Howell-Jolly bodies (leftover nuclear remnants) are not removed from the cytoplasm of erythrocytes, resulting in increased numbers of nucleated erythrocytes and erythrocytes containing Howell-Jolly bodies. The spleen plays an important role in the maturation of reticulocytes into erythrocytes; therefore, splenectomy can result in a reticulocytosis. Target cells are erythrocytes with too much membrane; normally, the spleen would remove the excess membrane. Thrombocytosis also occurs because normally, the spleen is a reservoir for one-third of the platelets in the body. Thrombotic thrombocytopenic purpura (TTP) is associated with increased numbers of reticulocytes (as a result of intravascular damage to erythrocytes); however, Howell-Jolly bodies and target cells are not normally seen in patients with TTP. Disseminated intravascular coagulation (DIC) is associated with an increase in reticulocytes due to destruction of erythrocytes by fibrin in the microvasculature, but again, target cells and Howell-Jolly bodies are not normally present. Sickle cell trait is not associated with any hematologic abnormalities. Autoimmune hemolytic anemias are associated with a pronounced reticulocytosis that is often accompanied by nucleated erythrocytes because the markedly stressed bone marrow is barely able to keep pace with the extravascular removal of erythrocytes. However, target cells are not usually present. |
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A 48-year-old woman has a large, fungating tumor involving her right breast. A simple
mastectomy is performed without axillary
dissection. Grossly, the tumor has a bulging surface with numerous cystic
spaces on cut section. Histologic sections show
a densely cellular stroma with normal ductal epithelium. Cystosarcoma phyllodes A 65-year-old woman has an
erythematous, scaly rash involving her left
nipple. The nipple is retracted. Painless lymphadenopathy is noted in the left lower axillary chain. PAGET DISEASE A 48-year-old woman has a
nonpalpable, 1.3-cm mass in the left upper outer
quadrant that was detected by mammography. The right breast was reported
as normal. On the mammogram, the left breast mass was described as having
an irregular border and a tightly clustered grouping of punctate microcalcifications. The mass is locally excised after
needle localization. Histologic sections reveal
terminal lobules distended by intermediate-size cells with scant mitotic
activity. Calcifications are noted in the acini.
Neoplastic cells infiltrate the stroma in an Indian-file pattern. Lobular carcinoma
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