Case Studies – Antimetabolites

00:00 Okay, let's go on to a question.

00:03 An HIV positive patient presents with an atypical pneumonia and was treated appropriately with Sulfamethoxazole/trimethoprim.

00:12 Which of the following organisms can be reasonably treated with this regimen in the HIV patient? Is it A) Pneumocystis.

00:21 Is it B) Toxoplasmosis.

00:24 Is it C) Microbacterium tuberculi.

00:28 It is D) A and B which is both Pneumocystis and Toxoplasmosis.

00:34 I've highlighted both of them there for you.

00:36 And or is it A and C which is Pneumocystis and Microbacterium tuberculi.

00:42 Good, you chose D which is A and B.

00:46 So trimethoprim/sulfamehtoxazole is an effective treatment for pneumocystis carinii pneumonia or PCP as well as toxoplasmosis gondii.

00:59 Both A and B are therefore correct.

01:01 So therefore the answer is actually D.

01:03 This drug combination acts against folic acid production.

01:07 It is not effective against viruses or against tuberculosis.

01:11 And it is rarely effective against fungal infections.

01:14 Now when you're doing an exam question like this, remember that most of the board exams have dropped the K-type exam question.

01:23 But some boards still use K-type exam questions.

01:26 It is still worthwhile for you to keep your skills answering this type of question.

01:31 But I will tell you that you probably will get this kind of K-type in the USMLE step one.

01:39 Okay, let's go on to another question.

01:41 Mr. Locutus recently had an implant.

01:44 He developed an infection that is composed of multiple gram positive cocci and gram negative rods.

01:50 He was started on ciprofloxacin and responded well.

01:54 Three weeks later he returns.

01:56 You are considering the same treatments.

01:58 Which is not a mechanism of resistance with this medication? Is it A) Point mutations in the gene coding for the 3OS ribosomal subunit.

02:11 Is it B) Changes in porins or pores in the cell membrane.

02:15 Is it C) Point mutations in the DNA gyrase enzyme.

02:19 And is it D) Enhanced ejection of the drug through new efflux mechanisms developed by organisms like Staphylococcus aureus.

02:30 You chose A.

02:32 Point mutations in the gene coding for the 3OS ribosomal subunit.

02:37 So ciprofloxacin is an excellent broad spectrum antibiotic.

02:40 It works well against a whole host of bacteria as well as some anaerobes both gram positives and gram negatives anaerobes.

02:48 These are the types of infections we see in prosthetic infections, tunnel infections and dirty procedures.

02:54 For example an emergency treatment of a motor vehicle accident.

02:59 Resistance develops through the following mechanisms.

03:02 Number 1, decreased intracellular accumulation due to changes in porins.

03:08 Number 2, Efflux mechanisms in microbacterium tuberculosis, staphylococcus aureus, staphylococcus pneumoniae.

03:16 And number 3, the changes in sensitivity of target enzymes like the DNA gyrase is also called topoisomerases via point mutations.

03:26 Now cipro does not act through the 30S ribosomal subunit.

03:30 The correct choice is therefore is A because that's the incorrect answer.