United States Medical Licensing Step 1

Section: Microbiology/Immunology
The NA inhibitors zanamivir and oseltamivir (tamiflu) were approved in 1999 for the treatment of influenza Aand influenza B. Amantadine (generic name) or Symmetrel (trade name) inhibits an early event in the multiplication cycle of influenza virus, as well as arenaviruses. It blocks the uncoating process. Mutations in the M protein genes result in the development of drug-resistant mutants. The drug is not used extensively in the United States because it seems impractical to control this type of infectious disease that is not ordinarily fatal. To protect individuals at high risk, and in those in whom the infection is potentially dangerous, the choice is between this drug and the influenza vaccine. In most cases, the vaccine is usually preferred. Acyclovir is used to shorten the duration of herpes simplex virus episodes and also to limit the duration of viral shedding (choice B). There are no antiviral drugs for rabies (choice D). Acyclovir has slight antiviral action for EBV (choice A). There is no antiviral agent against rhinovirus (choice E).

Section: Physiology
SGLT-1 is a sodium-dependent cotransport protein that uses the sodium electrochemical gradient to actively move glucose into the cell. GLUT2 (choice B) does transport glucose, but it is a facilitated diffusion carrier that moves glucose down its concentration gradient. The Na, K-ATPase (choice C) is a primary active transport protein. It uses the energy liberated from ATP hydrolysis to actively transport sodium and potassium. It establishes and maintains the sodium electrochemical gradient. The sodium: calcium countertransport protein (choice D, also termed NCX) uses the sodium electrochemical gradient to move calcium ions out of, not into, the cell. The calcium ATPase (choice A, also termed PMCA) is a primary active transport protein that actively transports calcium out of the cell. Both NCX and PMCA serve to maintain a low intracellular calcium concentration.

Section: Anatomy
Because the right side of the patient is affected, the stroke is in the territory of the left middle cerebral artery. This artery supplies the lateral aspect of the cerebral hemisphere, including portions of the pre-and postcentral gyri corresponding to the head, upper limb, and trunk on the primary motor (area 4 of Brodmann) and primary sensory (area 3,1,2 of Brodmann) cortical strips. These cortical control areas for the right lower limb are supplied by branches of the left anterior cerebral rtery (choice A), which is uninvolved in this case since the lower limb is intact. The left posterior cerebral artery (choice C) supplies the occipital and temporal lobes and is unaffected in this case. Since the right side of the brain controls the left side of the body and the patient is intact on the left side, none of the right side cerebral arteries (choices D and E) are involved.

Section: Physiology
Hyponatremia of this magnitude for any reason is likely to cause seizures and coma, especially if the deviation from normal occurred rapidly (e.g., over 1 or 2 days). Withdrawal from painful stimuli (choice A) is a normal reaction. If intact, it would have provided information on the intactness of sensation and motor reflexes on the tested side. However, lack of withdrawal would be consistent with either a structural or a metabolic cause of coma, and therefore does not help. Blood in the cerebral spinal fluid (choice B) in a nontraumatic lumbar puncture suggests a structural rather than a metabolic cause of coma. Pupillary light responses (choice C) are typically preserved in metabolic encephalopathies. Posturing of limbs (choice D) is suggestive of a focal process involving the brainstem or midbrain rather than a metabolic encephalopathy.

Section: Behavioral Science and Biostatics
The locus ceruleus, located in the pons, produces most of the noradrenergic input to the brain, and has receptors for opioids and autoreceptors for norepinephrine (?2) as well as GABA. The locus ceruleus seems to be involved in alertness and anxiety response. Choices A, C, D, and E refer to structures elsewhere.

Section: Anatomy
Capsular lesions of the corticobulbar system produce the "central seven" symptoms. Loss of the descending cortical fibers to the contralateral facial nucleus (cranial nerve VII) primarily affects the muscles of facial expression in the lower portion of the face, particularly those around the angle of the mouth and the nasolabial fold. Deviation of the protruded tongue (choice A), hypertonia/hyperreflexia (choice B), and spastic hemiplegia (choice E) are symptoms that result from a capsular lesion, but they would be seen contralateral to the affected capsule, and in this case would involve the left side of the body. Paraplegia (choice D) is not typically seen following a unilateral capsular lesion.

Section: Pathology and Path physiology
The physical description of this boy is that of Kleinfelter syndrome. This condition is not usually diagnosed until after puberty when the secondary sexual characteristics do not develop normally. Most of these individuals are 47, XXY but some may have additional X or Y chromosomes. 45, XO (choice A) is characteristic of a female with Turner syndrome which is usually diagnosed early in life. 45, YO (choice B) is not compatible with life and is not seen. 46, XX (choice C) is the chromosome number of a normal female and 46, XY (choice D) is that of a normal male. Males with 47, XYY (choice F) are above average in height and have an increased incidence of developing severe acne. IQ is normal but there may be mild learning disabilities.

Section: Biochemistry
Prolactin is necessary for initiation and maintenance of lactation. Physiologic levels act only on breast tissue primed by female sex hormones. Endocrine dysfunction leading to excessive prolactin production is associated with breast enlargement and impotence in males. Excessive production of corticotropin-releasing hormone (choice A) would result in an increase in adrenocorticotropic hormone (ACTH) production, which would lead to enhanced glucocorticoid and mineralocorticoid production. Excessive production of GnRH (choice B) would lead to increased production of luteinizing hormone (LH), follicle-stimulating hormone (FSH), and chorionic gonadotropin (hCG), with consequent effects on the female reproductive system. Excessive production of growth hormone (choice C) leads to gigantism if it occurs prior to epiphysial plate closure. If excessive release occurs following epiphysial plate closure acromegaly results, with characteristic facial changes (protruding jaw, enlarged nose) and enlarged feet, hands, and skull. Excessive production of melanocyte-stimulating hormone (choice D) would lead to hyperpigmentation of the skin.

Section: Biochemistry
PBG deaminase (also called hydroxymethylbilane synthase) catalyzes the heme biosynthesis reaction involving the head-to-tail condensation of four molecules of PBG to produce the linear tetrapyrrole intermediate, hydroxymethylbilane.
Hydroxymethylbilane can nonenzymatically cyclize into uroporphyrinogen I, which is why PBG deaminase is also known as uroporphyrinogen I synthase. ALA is the precursor for PBG, thus a defect in PBG deaminase would lead to excess ALA excretion. The compounds in choices BE all represent products of reactions that are downstream of PBG deaminase in the heme biosynthetic pathway and thus would not be excreted in high amounts in someone with AIP.

Section: Pathology and Path physiology Pemphigus vulgaris is an autoimmune disorder. The autoantibodies are directed against keratinocyte antigens with subsequent dyshesion and fluid-filled blister formation. Bacterial infections (choice B), dietary deficiencies (choice C), chemical toxins (choice D), and local ischemia (choice E) are not thought to be the causative agent of pemphigus vulgaris.