Once MSSA grows, trimethoprim-sulfamethoxazole gives way to cephalexin in this NU553 Unit 6 stewardship case, which also weighs his lisinopril and fixes a stop date on day five. Searches like "nu 553 unit 6 assignment example", "nu553 unit 6 sample" and "nu553 unit 6 example" land here.
The Culture Came Back MSSA: A Stewardship Case on What to Change After a Drained Thigh Abscess
[Student Name]
Purdue University Global
NU553: Advanced Pharmacology and Pharmacotherapeutics
Unit 6 Stewardship Case
[Instructor Name]
[Date]
The patient is a composite; susceptibilities and the local resistance rate are bracketed composite values.
1. The Empiric Decision
Mr. [O.], 54, supervises a mail sorting shift. He came to the clinic with a painful, fluctuant swelling on his left thigh, about 3 cm across, with a 2 cm rim of redness and warmth around it. He had no fever, and his heart rate and blood pressure were normal. The abscess was incised and drained, pus was sent for culture, and he was discharged on trimethoprim-sulfamethoxazole twice daily. His other medicines are lisinopril for hypertension and metformin for type 2 diabetes. His most recent potassium was [4.9] mmol/L, near the upper end of the normal range, and his kidney function was normal.
The empiric choice was reasonable at the time. Purulent skin infections are most often caused by Staphylococcus aureus, and in his community the proportion of those isolates resistant to methicillin was [composite] percent. For an abscess with surrounding cellulitis, the 2014 IDSA guideline supports incision and drainage and, when systemic signs or extensive surrounding cellulitis are present, an antibiotic active against MRSA (Stevens et al., 2014). Trimethoprim-sulfamethoxazole is one such agent, and in a trial of uncomplicated drained abscesses, it improved cure rates compared with placebo (Talan et al., 2016). The empiric decision is judged by what was known when it was made, and on that basis it stands.
2. The Culture
On day 2, the wound culture grew Staphylococcus aureus with the following susceptibilities:
1. Oxacillin: susceptible [composite]. The isolate is methicillin-susceptible, MSSA.
2. Cefazolin: susceptible [composite].
3. Trimethoprim-sulfamethoxazole: susceptible [composite].
4. Clindamycin: susceptible [composite], with a negative test for inducible resistance.
5. Doxycycline: susceptible [composite].
The organism is susceptible to the drug he is taking. That fact alone does not settle the question; stewardship asks whether he is on the best drug, not only an effective one.
3. The Revision
Three options were weighed, and each was asked the same three questions: is the spectrum wider than the organism requires, does the agent reach the likely pathogens, and what does it add to his existing medicines?
Option A: continue trimethoprim-sulfamethoxazole.
1. Spectrum: wider than needed. Its reason for being chosen, MRSA coverage, no longer applies.
2. Pathogens: effective against this MSSA. Its activity against beta-hemolytic streptococci, which often drive the surrounding cellulitis, is less reliable.
3. Interaction: trimethoprim shuts the sodium channels that line the distal nephron's collecting segment, reducing potassium excretion in the same way as the potassium-sparing diuretic amiloride. Lisinopril reduces aldosterone and also raises potassium. In older adults taking a renin-angiotensin system blocker, trimethoprim-sulfamethoxazole was associated with a substantially higher risk of hospital admission for hyperkalemia than amoxicillin (Antoniou et al., 2010). With a baseline potassium of 4.9 mmol/L and diabetes, which can impair potassium handling, that risk matters.
Option B: switch to cephalexin.
1. Spectrum: narrower. A first-generation cephalosporin active against MSSA and streptococci, with little activity against many gram-negative organisms.
2. Pathogens: covers both the cultured organism and the streptococci that may be involved in the cellulitis.
3. Interaction: no effect on potassium; no significant interaction with lisinopril or metformin.
Option C: stop antibiotics altogether.
1. Considered seriously. A small, well-drained abscess without surrounding cellulitis may need no antibiotic at all.
2. Rejected, with a stated reason: the 2 cm rim of cellulitis around this abscess is the indication for antibiotic treatment, and the trial evidence for drained abscesses favors treatment (Talan et al., 2016).
Decision: switch to cephalexin. It is narrower, it covers the organisms likely to be involved, and it removes a risk from his existing medication list. The potassium interaction is where the therapeutics course shows: a switch justified on spectrum alone is made doubly defensible because it also removes a hazard.
A potassium level is checked at the switch, since he has taken trimethoprim for two days. If it is raised, it is rechecked after the switch.
Explaining the Switch to Him
Mr. [O.] was already feeling better on day 2, and a call to say his antibiotic is changing could sound like a correction of a mistake. The call explains three things plainly. The lab has identified the germ, and it turned out to be the common, easier-to-treat type. The new tablet targets that germ more precisely and is less likely to disturb the helpful bacteria in his gut. And it avoids a known effect of the first antibiotic on potassium, which matters because his blood pressure tablet raises potassium too. He is asked to stop the old tablets, collect the new ones the same day and not finish the remaining trimethoprim-sulfamethoxazole later "just in case." Leftover antibiotics kept at home are a common source of self-treatment without a diagnosis.
Why Narrowing Matters Beyond This Patient
Narrowing therapy once a culture returns is one of the core actions of stewardship. Every day of an unnecessarily broad agent exerts selective pressure on the bacteria a person carries, in the gut, on the skin and in the nose, and favors organisms resistant to it. Broad agents also raise the risk of Clostridioides difficile infection and other disturbances of the normal flora. For a single patient, those effects are small and uncertain; across a clinic's prescribing over a year, they shape the local resistance rate that the next empiric decision will depend on. The bracketed MRSA rate in section 1 is, in part, a product of past prescribing.
The switch is recorded in the chart with its reason, so that a clinician who sees him at a follow-up visit knows the organism, its susceptibilities and why cephalexin was chosen. If he returns with a recurrence, that record will guide the next empiric choice.
4. Duration
Treatment will total five days, counted from the first dose of trimethoprim-sulfamethoxazole: two days completed and three days of cephalexin. For uncomplicated cellulitis, five days of treatment has been as effective as ten when the infection is improving (Hepburn et al., 2004), and the drainage has removed the source. His last dose falls on day 5.
A check on day 3, by phone or in person, asks whether the redness is receding, whether the wound is draining less and whether he has a fever. Treatment would be extended if the redness is still spreading at day 5, if a new collection forms, or if systemic signs appear. Otherwise, it stops on day 5.
Closing
The culture result changed the regimen, and so did his medication list. Stewardship in this case meant narrowing the drug, keeping the course short and fixing its end in advance, and it produced a safer choice for a man whose kidneys already manage his potassium with little margin.
References
Antoniou, T., Gomes, T., Juurlink, D. N., Loutfy, M. R., Glazier, R. H., & Mamdani, M. M. (2010). Trimethoprim-sulfamethoxazole-induced hyperkalemia in patients receiving inhibitors of the renin-angiotensin system: A population-based study. Archives of Internal Medicine, 170(12), 1045-1049. https://doi.org/10.1001/archinternmed.2010.142
Hepburn, M. J., Dooley, D. P., Skidmore, P. J., Ellis, M. W., Starnes, W. F., & Hasewinkle, W. C. (2004). Comparison of short-course (5 days) and standard (10 days) treatment for uncomplicated cellulitis. Archives of Internal Medicine, 164(15), 1669-1674. https://doi.org/10.1001/archinte.164.15.1669
Stevens, D. L., Bisno, A. L., Chambers, H. F., Dellinger, E. P., Goldstein, E. J. C., Gorbach, S. L., Hirschmann, J. V., Kaplan, S. L., Montoya, J. G., & Wade, J. C. (2014). Practice guidelines for the diagnosis and management of skin and soft tissue infections: 2014 update by the Infectious Diseases Society of America. Clinical Infectious Diseases, 59(2), e10-e52. https://doi.org/10.1093/cid/ciu296
Talan, D. A., Mower, W. R., Krishnadasan, A., Abrahamian, F. M., Lovecchio, F., Karras, D. J., Steele, M. T., Rothman, R. E., Hoagland, R., & Moran, G. J. (2016). Trimethoprim-sulfamethoxazole versus placebo for uncomplicated skin abscess. New England Journal of Medicine, 374(9), 823-832. https://doi.org/10.1056/NEJMoa1507476
How this NU553 Unit 6 example is structured
Stewardship is argued as a sequence of decisions, each judged with the information available when it was made, so the empiric choice is not condemned in hindsight. The revision section carries the weight. It lays the three options side by side and asks of each the stewardship questions: is the spectrum wider than the organism requires, does the agent reach the likely pathogens, and what does it add to the medicines he is already on? The potassium interaction is where the therapeutics course shows, since a narrower drug that also removes a risk from his existing list makes the switch doubly defensible. Stopping antibiotics entirely is considered seriously, because a small drained abscess may not need them, and rejected with a stated reason, the surrounding cellulitis. The duration section fixes the last day of treatment and what would extend it.
Get an NU553 Unit 6 example written to your instructions
Pass along the Unit 6 infection case exactly as NU553 set it, with any culture data, the patient's medication list and its rubric. A first sample costs nothing, takes 24-48h, and defends or revises the empiric choice with dated guidance, screens the replacement drug for conflicts with his current medicines, and ends on a named stop day. The paper above is an original model document written by our desk, not a submitted student paper and not an official Purdue University Global document.
NU553 Unit 6 questions, answered
Is it wrong to have started a broad agent empirically?
Not if the choice fit what was known at the time. Stewardship is about revising once results arrive, and cases in this course often reward a fair defense of the first decision followed by a clear narrowing. Criticizing an empiric choice only in hindsight suggests the author has confused a reasonable start with a mistaken one.
Why does the sample consider giving no antibiotic at all?
Because drainage cures many uncomplicated abscesses, and a stewardship case is stronger when every option, including none, is weighed. The sample declines that option for a specific reason drawn from the examination, surrounding cellulitis. If a case describes a small abscess with no spreading redness, the same reasoning could reasonably reach the opposite conclusion.
How should antibiotic duration be justified?
With a guideline and a stopping criterion. The sample cites the 2014 IDSA guidance supporting short courses for uncomplicated skin infections and ties the stop day to clinical signs at a set check. A duration chosen from habit, or a range with no rule for choosing within it, leaves the grader unable to see the reasoning. All durations in the sample are bracketed composites.