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Step 1 Casebook
Step 1 Casebook – Statins
Aug 10th
A woman rushes her 65-year-old husband into the emergency department after he said he felt crushing chest pain while eating a steak. Blood work is ordered and the physician finds that the man’s cholesterol is very high and that the man most likely had an acute myocardial infarction because of the finding of elevated enzymes. After consultation, the man is put on statins to lower his cholesterol level and told to avoid a fatty diet.
What are statins?
Statins are a class of hypolipidemic agents. They act to lower cholesterol by inhibiting 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase, the rate-limiting enzyme in cholesterol synthesis.
How do statins affect LDL, HDL, and triglyceride levels?
Statins decrease LDL levels greatly. They have been shown to mildly increase HDL levels and to decrease triglycerides as well.
What is the most effective agent for increasing HDL cholesterol levels?
Niacin has been shown to increase HDL better than most other drugs.
What agent is best suited for decreasing triglyceride levels?
A class of drugs known as “fibrates,” including gemfibrozil and clofibrate, has been shown to greatly diminish triglyceride levels.
What are some of the side effects of statins?
Side effects of this class of drugs include myositis and a reversible increase in liver function tests such as alanine aminotransferase and aspartate aminotransferase.
Why is it dangerous to combine statins and fibrates?
This drug combination significantly increases the risk of rhabdomyolysis.
By Rakesh Razdan Ahuja, class of 2010, Yale University School of Medicine; in association with Le TT, Takiar V, eds: First Aid Cases for the USMLE Step 1. New York: McGraw-Hill, 2009.
Step 1 Casebook – 4 year old boy with seizures
Aug 3rd
A 4-year-old boy is brought to the emergency department after being seen shaking on the floor. He had been playing with a video game when the episode began. He has no prior history of seizures or neurologic disorders. The episode resolved after 2 minutes.
What is the most likely diagnosis for this patient?
Most likely, the patient had a grand mal seizure due to the rapidly flashing lights in the video game.
What is the most appropriate treatment for this condition?
Grand mal or tonic-clonic seizures can be treated with phenytoin, carbamazepine, or valproic acid. In addition, avoidance of triggers can help prevent future seizures.
What are the side effects of the drugs associated with treatment?
Phenytoin is known to cause a characteristic gingival hyperplasia as well as a syndrome resembling systemic lupus erythematosus. Carbamazepine is known to cause blood dyscrasias such as aplastic anemia and agranulocytosis. Valproic acid commonly causes gastrointestinal upset and rarely, fatal hepatotoxicity.
If the boy were in class answering a question and suddenly stopped for 30 seconds, then continued with his explanation, what would be the diagnosis and treatment?
This is a classic case of an absence seizure, which is seen in children, but not in adults. The condition can be treated with ethosuximide. This drug acts by blocking thalamic T-type calcium channels.
Which two commonly used seizure medications may cause Stevens-Johnson syndrome?
Stevens-Johnson syndrome is a severe and life-threatening condition of the skin and mucous membranes. Its etiology is thought to involve the formation of immune hypersensitivity complexes. The two seizure medications associated with it are lamotrigine and ethosuximide.
By Rakesh Razdan Ahuja, class of 2010, Yale University School of Medicine; in association with Le TT, Takiar V, eds: First Aid Cases for the USMLE Step 1. New York: McGraw-Hill, 2009.
Step 1 Casebook – A newborn boy with apparent birth defects
Jul 27th
A newborn boy is found to have an increased head circumference, macroglossia, an umbilical hernia, lethargy, and a hoarse cry. The mother has no significant past medical history, takes no medications, and received adequate prenatal care. His serum thyroxine (T4) concentration is ~50% that of normal infants.
What is the most likely diagnosis?
Congenital hypothyroidism (aka cretinism).
Lack of adequate treatment of this condition is likely to result in what long-term consequence?
Congenital hypothyroidism is the most common treatable cause of mental retardation. Children with the condition were once popularly known as cretins (from the French word Chrétien or Christ-like), as they were so profoundly mentally retarded as to be considered incapable of sinning.
What are the most common causes of this condition?
Eighty-five percent are sporadic and 15% are inherited. The most common causes are thyroid dysgenesis (ectopy, hypoplasia, or complete agenesis); disordered thyroid hormone synthesis, secretion, or transport; and iodine deficiency. The condition is also more common in places where goiter is endemic.
How might thyroid disease in the mother affect the developing fetus?
In mothers with autoimmune hypothyroidism, thyroid-stimulating hormone-receptor blocking antibodies can cross the placenta and affect the fetus’ thyroid function. This “transient congenital hypothyroidism” should resolve within 1–3 months, when maternal antibodies are cleared from the infant’s system. Hyperthyroid women are more likely to have difficulties conceiving and have higher rates of miscarriage. Hyperthyroidism must be controlled in pregnancy, as the risks of miscarriage and birth defects are much higher without treatment.
Which thyroid medications are safe in pregnancy?
Thyroid hormone replacements (such as levothyroxine) for hypothyroidism are safe in pregnancy, given that the medication is appropriately prescribed and consumed. Antithyroid drugs given to hyperthyroid mothers can cross the placenta. Propylthiouracil (PTU) is the drug of choice for treating hyperthyroidism in pregnancy, as its transplacental effects on the fetus are less severe. PTU is cleared from the infant’s system within a few days after birth. Methimazole readily crosses the placenta and should be avoided in pregnancy, as should medications with large quantities of iodine, such as amiodarone for cardiac arrhythmia.
Step 1 Casebook – 56-year-old man with a productive cough
Jul 20th
A 56-year-old homeless man with a history of alcohol abuse is found unconscious on a street corner. He is brought into the emergency department for monitoring. He is found to have a temperature of 38.9°C (102°F). X-ray of the chest is significant for pleural effusion, empyema, and cavitation of his right upper lobe. As he regains consciousness, he complains of chills and begins to cough up thick, bloody sputum. When stained, the sputum reveals gram-negative rods.
What is the most likely diagnosis?
Pneumonia secondary to infection with Klebsiella pneumoniae.
What are the characteristics of the causative organism?
Klebsiella pneumoniae is a gram-negative rod with a prominent polysaccharide capsule. This capsule helps protect it from host defenses such as phagocytosis and complement activity. On its surface, it has an O antigen, which is a lipopolysaccharide, and a K antigen, which is associated with the capsule. It also has adhesins so it can bind to host cells. It is part of the normal gastrointestinal (GI) flora in many people.
How does this patient’s infection usually present and which populations are most at risk?
Pneumonia caused by Klebsiella pneumoniae commonly presents with high fever, chills, flu-like symptoms, and a productive cough. The sputum is often described as being like currant jelly because it is thick, bloody, and mucoid. The pneumonia is typically a lobar one with necrosis and the formation of cavitary lesions in the affected lobe. There are usually signs of abscess formation, empyema, pleural adhesions, and pleural effusions on x-ray. Because this bacterium is often part of the normal GI flora, pneumonia is often secondary to aspiration. As a result, the susceptible populations are those with a history of loss of consciousness, such as alcoholics or stroke victims. It is also more common in those with diabetes and chronic obstructive pulmonary disease.
What other infections are most commonly seen with this organism?
K. pneumoniae is a common cause of nosocomial infections such as urinary tract infections (especially in those with a Foley catheter), thrombophlebitis, sepsis, liver abscesses, and meningitis.
What is the most appropriate treatment for this condition?
K. pneumoniae is resistant to various antibiotics via a plasmid-mediated mechanism. Antibiotics that typically work are the third-generation cephalosporins, carbapenems, aminoglycosides, and quinolones. However, in Klebsiella pneumonia, mortality reaches 50%, even with antibiotics.
By Christina L. Shenvi, PhD, class of 2009, Yale University School of Medicine; in association with Le TT, Takiar V, eds: First Aid Cases for the USMLE Step 1. New York: McGraw-Hill, 2009.
Step 1 Casebook – Drugs used to treat hypertension and heart failure
Jul 13th
A third-year clerk on medical rounds notices that a large majority of his patients have either hypertension or congestive heart failure. He realizes that in treating both of these conditions, one class of drugs is commonly used.
What class of drugs is commonly used for these two conditions? List examples of drugs within this class?
These two conditions are both treated with B-blockers. Some commonly used B-blockers include propranolol, metoprolol, atenolol, esmolol, and timolol, among others. These agents all have in common the ending “–olol.”
For what other conditions is this class of drugs useful?
B-Blockers can be used to treat hypertension by decreasing cardiac output and renin secretion. After myocardial infarction, B-blocker use has been shown to decrease mortality. For angina pectoris, B-blockers are helpful because they decrease both heart rate and contractility, leading to a decrease in oxygen demand. Finally, B-blockers such as timolol can be used in glaucoma to inhibit the secretion of aqueous humor.
What is the selectivity of this class of drugs?
Both selective and nonselective B-blockers exist. Nonselective B-blockers block both B1 and B2 receptors equally and include propranolol, timolol, and nadolol. B1-Selective blocking agents can be remembered with the mnemonic “A BEAM of B1-blockers,” and include Acebutolol, Betaxolol, Esmolol, Atenolol, and Metoprolol.
What are some of the toxicities associated with this class of drugs?
Toxicity of B-blockers is significant. Clinical signs of toxicity include impotence, sleep alterations, and bradycardia.
Which two specific patient populations should be monitored closely while receiving treatment with B-blockers?
Caution should be used when giving B-blockers to diabetics because they may mask the usual symptoms of hypoglycemia. In patients with asthma, there is a risk of exacerbation of bronchiolar constriction due to antagonism of B2 receptors.
By Rakesh Razdan Ahuja, class of 2010, Yale University School of Medicine; in association with Le TT, Takiar V, eds: First Aid Cases for the USMLE Step 1. New York: McGraw-Hill, 2009.
Step 1 Casebook – 47-year-old man presents for an annual check-up
Jul 6th
A 47-year-old man presents to his physician for his annual check-up. He reports that he does not smoke, consumes a reasonable diet, exercises regularly, and drinks less than one drink per week. He does not take any medications or supplements. Family history is notable for the death of his father at age 55 years from a “heart attack,” and an older sister who had recently undergone a balloon angioplasty. Physical examination is significant for normal blood pressure and a body mass index (BMI) of 29 kg/m2. His thyroid and renal function tests are normal. His screening lipid profile is as follows:
Fasting triglycerides (TG): 450 mg/dL
Total cholesterol: 220 mg/dL
LDL: 160 mg/dL
HDL: 30 mg/dL
What are the results of the screening lipid profile?
Both hypertriglyceridemia (>150 mg/dL) and hypercholesterolemia (>200 mg/dL) are present. His “Lousy” LDL cholesterol is elevated (>100 mg/dL), whereas his “Helpful” HDL cholesterol is below optimal (<40 mg/dL for men). The family history of coronary artery disease (CAD) raises concerns for a familial dyslipidemia. Secondary causes of dyslipidemia, such as nephrotic syndrome and hypothyroidism, are unlikely in this otherwise healthy person.
What is the classification scheme of familial dyslipidemia?
Type I: Hyperchylomicronemia: Increased chylomicrons (CMs), TG, and cholesterol
Type IIa: Hypercholesterolemia: Increased LDL and cholesterol
Type IIb: Combined hyperlipidemia: Increased LDL, very-low-density lipoprotein (VLDL), TG, and cholesterol
Type III: Dysbetalipoproteinemia: Increased intermediate-density lipoprotein (IDL), VLDL, TG, and cholesterol
Type IV: Hypertriglyceridemia: Increased VLDL and TG
Type V: Mixed hypertriglyceridemia: Increased VLDL, chylomicrons, TG, and cholesterol
What is the function of lipoproteins?
Lipoproteins are large spherical complexes containing a hydrophobic core of TG and cholesteryl esters, and a hydrophilic shell of phospholipids and unesterified cholesterol, that serve to transport lipids throughout the body. LDL and HDL carry a significant amount of cholesterol: LDL from the liver to the periphery and HDL from the periphery to the liver.
What happens to dietary lipids after absorption from the intestinal lumen?
- Absorption of dietary TGs requires hydrolysis by lipases within the gut lumen into glycerol and fatty acids. These breakdown products move into the enterocytes.
- Enterocytes reform TGs from molecules of free fatty acids (FFAs) and glycerol, and esterify dietary cholesterol and retinol.
- TGs and esters are combined with apolipoproteins (apo) and phospholipids into CMs.
- CMs are delivered to the circulation via the lymphatics.
- Lipoprotein lipases on endothelial cell surfaces (with help from cofactor apo-C-II on CMs) hydrolyze TGs into FFAs for delivery to tissues; cholesterol esters are transferred to HDL.
- CM remnants are removed by the liver, in a process requiring apo-E.
What is the class of medications that inhibits the rate-limiting step of cholesterol biosynthesis?
The rate-limiting step in cholesterol biosynthesis is the conversion of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) to mevalonic acid. Inhibition of this step by HMG-CoA reductase inhibitors, also known as statins, results in greater hepatic LDL receptor turnover, accelerated clearance of LDL, and ultimately decreased hepatic synthesis of LDL cholesterol.
By Kelvin Lau, MD, Resident, Department of Pediatrics, Children’s Hospital of Philadelphia; in association with Le TT, Takiar V, eds: First Aid Cases for the USMLE Step 1. New York: McGraw-Hill, 2009.
Step 1 Casebook – 70-year-old diabetic man with benign prostatic hyperplasia
Jun 29th
A 70-year-old man with type 2 diabetes mellitus (DM) and benign prostatic hyperplasia (BPH) is rushed to the emergency department after being found by his weekly visiting nurse to be lethargic, disoriented, and barely conscious. She found that his pill box was full of last week’s diabetes and BPH medications. On physical examination, the patient has a Glasgow Coma Scale score of 8 (out of 15), a fever of 38.9°C (102° F), hypotension, and tachycardia. Abdominal examination reveals suprapubic fullness. The physician is unable to elicit costovertebral angle tenderness secondary to his poor mental status, but Foley catheterization reveals 2 L of cloudy, putrid urine. Initial chemistries on venous blood sampling reveal serum glucose of 1100 mg/dL and pH of 7.4.
What is the most likely diagnosis?
Urinary tract infection/pyelonephritis complicated by hyperosmolar hyperglycemic nonketotic coma (HHNK), secondary to medication noncompliance and possible sepsis. HHNK frequently occurs in elderly patients taking oral hypoglycemic agents who are inadequately monitored, or patients with previously undiagnosed type 2 DM. This hyperglycemic crisis is a medical emergency, with a 10%–20% mortality rate.
What are the primary features of this condition?
Severe hyperglycemia; serum glucose is frequently >1000 mg/dL, unlike diabetic ketoacidosis (DKA) (<800 mg/dL) (see comparison below)
- No ketosis, pH normal
- Profound dehydration
- Neurologic symptoms
What are some symptoms associated with this condition?
Patients with HHNK often present late, with symptoms of hyperosmolality. It is not uncommon for a person to present in coma. Other neurologic derangements include:
- Drowsiness and lethargy
- Delirium
- Seizures
- Visual changes/disturbances
- Hemiparesis
- Sensory deficits
Patients with DKA often present earlier than patients with HHNK do, as they develop symptoms of ketoacidosis (ie, Kussmaul’s respirations and abdominal pain).
Why is the patient not acidotic (pH <7.4)?
Patients with type 2 diabetes have sufficient insulin production to prevent ketoacidosis from occurring first. Conversely, in DKA extremely low insulin causes triglycerides to be converted to fatty acids, which are then converted to ketoacids in the liver.
What treatment is most appropriate for this condition?
The primary goal of therapy is rehydration and correction of electrolyte abnormalities. Patients may need over 10 L of intravenous fluids, but be careful in the elderly! Insulin is administered to promote normal glucose metabolism. It is also important to treat the underlying condition (in this case, urinary tract infection and pyelonephritis). Remember, the elderly are very sensitive to infection, but rarely present typically. An elderly patient presenting with acute mental status changes should be worked up fully for an infectious cause, among others (eg, cerebrovascular accident). Patient education is crucial, as is access to social services (ie, more frequent Visiting Nurse Association services versus placement in a managed care facility).
Comparison of DKA With HHNK:
Diabetic Ketoacidosis (DKA):
- Frequently <40 years
- Duration of symptoms ~2 days or less
- Glucose level <800 mg/dL
- Sodium Normal or low
- Potassium Varies
- Bicarbonate Low
- pH – Acidemia present
- Serum osmolality <350 mOsm
- Urine ketones Present (4+)
- Mortality rate 2%–10%
Hyperosmolar Hypoglycemic Non-Ketotic Coma (HHNK):
- Frequently <60 years Duration of Symptoms Frequently >5 days
- Glucose Level >800 mg/dL
- Sodium Normal or High
- Potassium Varies
- Bicarbonate Normal
- pH – Acidemia absent
- Serum Osmolality >350 mOsm
- Urine Ketones Minimal to none (<2+)
- Mortality rate 10%–20%
Step 1 Casebook – 35-year-old nurse with heart palpitations
Jun 22nd
A 35-year-old nurse presents to the emergency department complaining of heart palpitations. She has also been jittery, hot, and sweaty for several hours. She has lost 15 lb (6.8 kg) in the past 2 months, missed her period in the last month, and has had frequent bouts of diarrhea ever since her fiancé cheated on her 3 months ago. She has no past medical history and denies medication or drug use. Physical examination reveals an anxious, warm, and diaphoretic woman with increased deep tendon reflexes and a fine tremor. Head and neck examinations are unremarkable. Her laboratory findings are as follows:
Serum thyroxine (T4): 30 µg/dL (normal 4–12 µg/dL)
Free thyroxine (FT4): 4.8 ng/dL (normal 0.7–1.9 ng/dL)
Serum triiodothyronine (T3): 270 ng/dL (normal 80–180 ng/dL)
Thyroid-stimulating hormone (TSH): 0.04 µU/mL (normal 0.5–6 µU/mL)
Thyroxine-binding globulin (TBG): 2 µg/dL (normal 12–20 µg/dL)
What is the most likely diagnosis?
Self-administration of thyroxine to induce factitious hyperthyroidism. This condition is seen in people with factitious disorders (feigning illness to play the sick role), malingering (feigning illness to achieve secondary gain), or those trying to lose weight. Health care workers and caretakers of chronically ill patients are at risk of abusing regularly prescribed drugs, particularly pain medication, insulin, and thyroid hormone.
How is thyroid hormone synthesis regulated?
The hypothalamus secretes thyroid-releasing hormone (TRH), which acts on the anterior pituitary gland to secrete TSH. TSH stimulates the thyroid gland to release T3 and T4. T4 is secreted in greater abundance, whereas T3 is the more active form. T4 is converted to T3 in the periphery. In the circulation, thyroid hormones are bound to TBG, a protein produced by the liver. Only free/unbound thyroid hormone is active. Levels of T3 and T4 in the periphery negatively feedback on the hypothalamus and pituitary, to suppress TRH and TSH, respectively.
What is a possible explanation for the patient’s laboratory results?
The patient has increased levels of free thyroid hormone (T3/T4) through ingestion. (Synthetic thyroid hormone is available in different combinations of T3 and T4.) She has decreased TSH levels, due to the negative feedback by the elevated levels of T3 and T4. She has decreased free TBG levels, as the binding sites have been saturated by the increased thyroid hormones.
How do the physical examination findings aid in diagnosing this condition?
While this patient is clearly displaying signs and symptoms of hyperthyroidism, the fact that she has no goiter, no exophthalmos, and no pretibial myxedema, makes Graves’ disease less likely.
How can this condition be distinguished from a panic attack?
There clearly is a psychological component to this patient’s disorder. However, on the boards, anxiety attacks are typically short-lived, peak within 5–10 minutes, and normally resolve within 30 minutes.
What would be expected on a radioactive iodine (RAI) scan?
Decreased RAI uptake. The normal hypothalamic-pituitary-thyroid axis is intact in this case. Endogenous thyroid function is suppressed by negative feedback, and therefore will not take up the RAI as avidly. In contrast, in Graves’ disease the RAI would show increased uptake, as the negative feedback mechanism is perturbed secondary to thyroid-stimulating immunoglobulins against the TSH receptor.
Step 1 Casebook – 42-year-old accident victim with potential aortic dissection
Jun 15th
A 42-year-old man suffers a high-speed motor vehicle collision with a tree and police quickly respond to the scene to find the man with an extremely faint pulse. The man is rushed to the emergency department. The patient is slipping in and out of consciousness, and the attending physician quickly gives the man hexamethonium because she believes the man has an aortic dissection.
What kind of drug is hexamethonium?
Hexamethonium is a ganglion blocking agent. It is used to block postganglionic transmission such as in a vagal response to a change in blood pressure. Although this drug is rarely used now due to its lack of specificity, it is still administered in cases of emergency such as aortic dissection.
What receptor does hexamethonium target?
Hexamethonium is a nicotinic acetylcholine (ACh) receptor antagonist.
What are the names of some other important ganglion blocking agents?
Other ganglion blockers include mecamylamine, trimetaphan, and pentolinium.
By Rakesh Razdan Ahuja, class of 2010, Yale University School of Medicine; in association with Le TT, Takiar V, eds: First Aid Cases for the USMLE Step 1. New York: McGraw-Hill, 2009.
Step 1 Casebook – 56-year-old valve replacement patient with splinter hemorrhages
Jun 8th
A 56-year-old man presents to the emergency department with fevers, chills, and a feeling of malaise. He states that the onset of symptoms was gradual. His past medical history is significant for replacement of his aortic valve 6 months ago. On physical examination, there is a 2/6 systolic ejection murmur that was not noted previously. Splinter hemorrhages are present in his nailbeds.
What is the most likely diagnosis?
Infective endocarditis caused by Staphylococcus epidermidis. Although other bacteria can cause endocarditis, the gradual onset and the presence of a prosthetic valve make S. epidermidis the most likely pathogen.
What are the characteristics of the infective agent in this diagnosis?
S. epidermidis colonizes the skin of most people. It is a gram-positive coccus that is catalase-positive, but coagulase-negative. It is non-motile and non-spore-forming. It is known to make an exopolysaccharide that is slimy and allows it to adhere to foreign bodies and prevent phagocytosis.
What are the risk factors for this condition?
In general, S. epidermidis is a bacterium with low virulence and infectivity. It is known for its ability to colonize foreign bodies. Hence the main risk factor for infection is the presence of prosthetic devices, such as prosthetic joints and heart valves, vascular shunts, grafts, and catheters.
How is this condition diagnosed?
The diagnosis is made by culture. When cultures come back positive, it is usually a contamination from the skin; true-positives are only seen 4%–12% of the time. For this reason, it is recommended that repeat cultures be done at different times. It is also important not to start antibiotic treatment before cultures are drawn, or else the pathogen might not be present in sufficient quantities to culture. An alternative is to run polymerase chain reaction on the sample to see if the bacteria are all of one strain. If they are, it is likely an infection. An echocardiogram can be done to look for vegetations on heart valves.
What is the treatment for this condition?
S. epidermidis is resistant to most antibiotics, including methicillin and the b-lactams. For this reason, the antibiotic used for treatment is usually vancomycin. In many cases, antibiotics will not be enough to eliminate the infection, and surgical removal of the prosthesis will be required.
By Christina L. Shenvi, PhD, class of 2009, Yale University School of Medicine; in association with Le TT, Takiar V, eds: First Aid Cases for the USMLE Step 1. New York: McGraw-Hill, 2009.
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