USMLE Step 1 - Practice Answers

[nicko18]

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B

Pyridoxine, or vitamin B6, is sometimes depleted with isoniazid use. Patients with pyridoxine deficiency may experience neurologic symptoms such as convulsions and fasciculations. The treatment of this disorder is slow intravenous administration of 2.5g of pyroxidine.

 

[nicko18]

D

Premeditation with a plan of lethality is a major factor in successful completion of a suicide attempt. In addition, individuals on more than two prescription medications, those who live alone, and those with a history of alcohol or drug use are at risk for completed suicide.

 

[nicko18]

D

Since Tay-Sachs is lethal in childhood, the only individuals carrying the gene are heterozygotes. The only mating that produces progeny with disease will be mating between two heterozygotes. Using blood test data, the probability that a random individual is a heterozygote is 0.1. The probability that two heterozygotes will produce a child with Tay-Sachs disease is 0.25. So the overall probability that two individuals in the population will produce a child with Tay-Sachs is (0.1)(0.1)(0.25) or 0.0025.

 

[nicko18]

E

Chronic use of both alcohol and sedatives can stimulate the P-450 mixed oxidase system, which is responsible for the metabolic degradation of many natural substances and drugs. This system is located on the smooth endoplasmic reticulum, which proliferates in response to chronic stimulation. This expansion of the smooth endoplasmic reticulum can be significant, both from a structural and metabolic perspective. Ultrastructurally, the hepatocyte cytoplasm becomes nearly completely packed with smooth endoplasmic reticulum, while other organelles are pushed into the small amounts of remaining cytoplasm. Metabolically, the ability to degrade toxic substances is markedly enhanced, which manifests as the higher sedative dose required to induce sleepiness in the non-drinking alcoholic. However, a prominent addictive effect of sedatives occurs with alcohol. If any person (including the alcoholic in question) takes a large dose of certain sedatives while drinking, the P-450 system is saturated with alcohol, and thus cannot effectively degrade the sedative, leading to severe respiratory depression and possibly death.

 

[nicko18]

D

Karyorrhexis refers to a pattern of nuclear degradation in which a pyknotic or partially pyknotic nucleus undergoes fragmentation followed by complete lysis. This pattern is common in the neutrophils present in acute inflammation.

 

[nicko18]

E

The patient described has staphylococcal arthritis, most likely a result of intravenous drug abuse, which can introduce Staph. aureus into the bloodstream, producing bacteremia and septic arthritis. Infectious arthritis, especially when acute, must be treated as a medical emergency to avert joint destruction and/or septicemia. The joint should be tapped, cultures sent for both aerobic and anaerobic analysis, and a TB test planted. Appropriate antibiotic treatment must be instituted immediately.