NBME 24 Section 1 Questions 1-50

A 40-year-old woman with a 25-year history of type 1 diabetes mellitus has a serum creatinine concentration of 2.5 mg/dl. A creatinine clearance test is ordered to determine the glomerular filtration rate. This test is limited, compared to an inulin clearance test, because of which of the following properties of creatinine?

• Actively pumped into the distal convoluted tubule

• Crosses freely into the loop of Henle

• Reabsorbed into the collecting duct

• Secreted by the proximal tubule

Answer explanation

Creatinine clearance overestimates GFR because creatinine is excreted by tubules. The afferent arteriole brings many nutrients and substances to the glomerulus, where filtration happens based on pressures. Then along the tubule, some things are reabsorbed (all sugar, proteins, etc in PCT), while some things are secreted (PAH). Everything that was filtered and secreted gets excreted (clearance). Inulin is neither reabsorbed nor secreted so it is freely filtered and best indicates GFR. Creatinine is secreted by the kidney, so it overestimates GFR.

A 10-year-old boy who was adopted from the Democratic Republic of Congo 2 weeks ago is brought to the physician for an initial examination. He appears slim, has thin extremities, and is in no distress. His temperature is 37.3°C (99.1°F), pulse is 100/min, respirations are 20/min, and blood pressure is 118/68 mm Hg. The lungs are clear, and heart sounds are normal. The abdomen is soft and nontender, and there is no hepatosplenomegaly. There are four 0.5-mm, firm, nontender nodules: two over the right iliac crest, one on the left thigh, and one on the left knee. Examination of a skin snip from one of the nodules shows microfilariae.

• Black fly

• Body louse

• Culex species mosquito

• Ixodes species tick

• Reduviid bug

Answer explanation

This patient has Onchocerca volvulus by the female black fly. The other answer choices are generally related to bacteria or unrelated to any nematodes. This child is presenting with practically no symptoms besides the four black skin nodules indicative of O volvulus.

A 3-year-old girl is brought to the physician for a well-child examination. She has had a flat purplish lesion that measures 2.1 cm in diameter on the right side of her face since birth. The lesion shows large cavernous vascular channels. Which of the following is the most likely outcome of the lesion?

• Concurrent congenital heart disease

• Local invasion

• Malignant degeneration

• Necrosis and scarring

• Spontaneous regression

Answer explanation

The lesion described is likely a hemangioma, which often undergoes spontaneous regression over time, especially in children. This is the most common outcome, unlike the other options which are less likely in this context.

This is not a Strawberry Hemangioma, which would be raised or nodular. But if you did think it is a strawberry hemangioma, then that also regresses. This patient has a nevus simplex. Another similar lesion is a stork bite (port-wine stain). These lesions disappear/regress by age 5-6 years.

At 2:00 AM, after sleeping for 5 hours, a 32-year-old man with type 2 diabetes mellitus has a decrease in serum glucose concentration to 37 mg/dl. In response to the hypoglycemia, cortisol secreted by the adrenal cortex induces the synthesis of which of the following enzymes in the adrenal medulla?

• Acetyl-CoA carboxylase

• Homocysteine methyltransferase

• Methionine adenosyltransferase

• Methylmalonyl-CoA racemase

• Phenylethanolamine N-methyltransferase

Answer explanation

This question is a simple biochemistry question regarding mechanisms of managing hypoglycemia. Cortisol is one of the defenses against hypoglycemia. The question asks for an enzyme in the adrenal medulla, which synthesizes NE and Epi. Choice E is the only relevant enzyme. It works with SAM to convert NE to Epi and is upregulated by cortisol. You should know exactly what the other enzymes are for. Acetyl-CoA carboxylase is the first enzyme for fatty acid synthesis and takes Acetyl CoA and yields Malonyl CoA. Choice B is related to homocysteine becoming methionine and is related to homocystinuria. Choice C is a relevant enzyme because it makes the SAM that PNMT uses it is not upregulated by cortisol. Choice D, a "racemase" is probably an enzyme youve never heard of. It is an epimerase involved in fatty acid and amino acid breakdown.

A 25-year-old woman takes an overdose of barbiturates and is found unresponsive several hours later by her husband. She is comatose and undergoes endotracheal intubation and placement on a mechanical ventilator. Three weeks later, she opens her eyes spontaneously and has roving conjugate eye movements. She has no purposeful responses to stimuli. Brain stem reflexes remain intact. An EEG is diffusely slow without evidence of cortical reactivity. The patient has signed a living will and, consistent with its directives, her husband requests that mechanical ventilation be discontinued. Which of the following is the most appropriate next step in patient care?

• Extubate the patient and discontinue mechanical ventilation; do cardiopulmonary resuscitation if her heart stops beating

• Extubate the patient and discontinue mechanical ventilation; make no attempt to do cardiopulmonary resuscitation in case of cardiac or respiratory failure

• Extubate the patient and discontinue mechanical ventilation; reintubate the patient if spontaneous respirations cease

• Obtain a court order to maintain mechanical ventilation

• Refer the case to the hospital ethics committee

Answer explanation

Very simple and straightforward. Patient is unresponsive, has no EEG activity, and no purposeful movement. The question states "consistent with its directives" so her husband is simply requesting what the patient wanted. The patient does not have a DNI or DNR since she was intubated. She must just be allowed to die rather than sustaining her body alive.

A 63-year-old woman undergoes operative repair of a leaking berry aneurysm in the circle of Willis. Two days later, a CT scan of the head shows a widening of the subarachnoid space. Which of the following changes in cerebrospinal fluid is the most likely cause of this finding?

• Decreased absorption by the choroid plexus

• Decreased movement through the arachnoid villi

• Decreased movement through the cerebral aqueduct

• Increased absorption by the choroid plexus

• Increased movement through the arachnoid villi

Answer explanation

This patient has a communicating hydrocephalus, as is evident by the fact that there is no obstruction that is impeding the CSF flow. The patient has a widening of the subarachnoid space because the arachnoid granulations are not absorbing the CSF, most likely due to inflammatory injury. Blood is inflammatory. Her leaking aneurysm has caused inflammation and damaged the arachnoid granulations. The arachnoid granulations absorb CSF and send it to the dural venous sinuses, which empty into the internal jugular vein. The other choices also do not make much sense. The choroid plexus does not absorb CSF, it produces CSF. Choice C is indicative of a noncommunicating (obstructive) hydrocephalus.

A 23-year-old woman is brought to the emergency department because of shortness of breath for 2 weeks. Her respirations are 28/min. Physical examination shows no other abnormalities. Laboratory studies show: Serum: • Na+ 135 mEq/L • K+ 4.0 mEq/L • Cl- 110 mEq/L • HCO3- 15 mEq/L • Arterial pH on room air 7.25 Urine: • Sodium 20 mEq/L • Chlorine 30 mEq/L • Potassium 15 mEq/L Which of the following is the most likely diagnosis?

• Crohn disease

• Lactic acidosis

• Alcoholic ketoacidosis

• Diabetic ketoacidosis

• Renal tubular acidosis

Answer explanation

The patient has metabolic acidosis (low HCO3- and pH) with a normal anion gap, indicated by urine sodium and chloride levels. This is consistent with renal tubular acidosis, where the kidneys fail to excrete acids properly.

A 30-year-old woman comes to the physician because of a 2-week history of right flank pain. Her blood pressure is 168/98 mm Hg. Physical examination shows a 7-cm mass that is palpated in the right side of the abdomen. A CT scan shows a mass in the retroperitoneal space on the right side that is compressing the renal artery. If the perfusion pressure to the affected kidney is decreased, but glomerular filtration rate and renal plasma flow remain unchanged, which of the following mechanisms mediates the autoregulation in the renal arterioles in this patient?

• Decreased afferent and efferent arteriolar resistance

• Decreased afferent arteriolar resistance

Answer explanation

In response to decreased perfusion pressure, the kidney autoregulates by decreasing afferent arteriolar resistance, allowing more blood flow to maintain glomerular filtration rate, hence the correct choice is decreased afferent arteriolar resistance.

A male newborn is delivered in the hospital at 40 weeks' gestation to a 31-year-old primigravid woman following an uncomplicated spontaneous vaginal delivery. The mother immigrated to the USA from Brazil at 20 weeks' gestation. At 8 weeks' gestation, she was evaluated because of a 1-month history of low-grade fever, marked fatigue, and diffuse lymphadenopathy. Her symptoms resolved spontaneously, and she received no treatment. At that time, test results for Epstein-Barr virus and cytomegalovirus were negative. The mother's only medication was a prenatal vitamin. The newborn is 33 cm (13 in; 25th percentile) long and weighs 2438 g (5 lb 6 oz; 35th percentile); head circumference is 31 cm (12 in; 3rd percentile). Temperature is 37.0°C (98.6°F), pulse is 120/min, respirations are 18/min, and blood pressure is 80/50 mm Hg. Eye examination shows chorioretinitis. CT scan of the head shows hydrocephalus and cranial calcifications. This newborn's congenital infection was most likely acquired by the mother via which of the following modes of transmission?

• Contact with fomites

• Ingestion of undercooked meat

• Inhalation of air droplets

• Kissing

• Tick bite

Answer explanation

The newborn's symptoms suggest congenital toxoplasmosis, commonly acquired by the mother through ingestion of undercooked meat containing Toxoplasma gondii. Other transmission modes listed are less relevant for this infection.

A 53-year-old woman comes to the physician because of a 2-week history of fainting precipitated by exercise and intermittent substernal chest pain that is relieved by rest. Her blood pressure is 120/80 mm Hg. Cardiac examination shows a prominent left ventricular impulse. A late-peaking, harsh midsystolic murmur is heard best at the second right intercostal space with radiation to the carotid arteries bilaterally. An ECG shows evidence of left ventricular hypertrophy. Angiography shows no coronary artery lesions. Which of the following is the best explanation for the syncope?

• Coronary artery compression by hypertrophic left ventricular myocardium

• Coronary artery spasm

• Fixed cardiac output in spite of increased demand

Answer explanation

The patient's symptoms and findings suggest fixed cardiac output due to left ventricular hypertrophy, which limits the heart's ability to increase output during exercise, leading to syncope.