A cell biologist is studying the activity of a novel chemotherapeutic agent against a cancer cell line. After incubation with the agent and cell detachment from the tissue culture plate, the DNA is harvested from the cells and run on a gel. Of note, there are large bands at every multiple of 180 base pairs on the gel. Which of the following explains the pathophysiology of this finding?
Q192
A 39-year-old man comes to the physician for a follow-up examination. He was diagnosed with latent tuberculosis infection 3 months ago. He has had generalized fatigue and dyspnea on exertion for the past 6 weeks. He does not smoke and drinks 2–3 beers on weekends. Vital signs are within normal limits. Examination shows conjunctival pallor. Laboratory studies show:
Hemoglobin 7.8 g/dL
Mean corpuscular volume 72 μm3
Red cell distribution width 17% (N = 13–15)
Reticulocyte count 0.7%
Leukocyte count 6,800/mm3
Platelet count 175,000/mm3
Serum
Creatinine 0.8 mg/dL
Iron 246 μg/dL
Ferritin 446 ng/mL
Total iron-binding capacity 212 μg/dL (N = 250–450)
Which of the following is the most likely cause of this patient's symptoms?
Q193
A 65-year-old G4P4 woman presents to her primary care physician complaining of a breast lump. She reports that she felt the lump while conducting a breast self-examination. Her past medical history is notable for endometrial cancer status post radical hysterectomy. She takes aspirin and fish oil. The patient drinks 3-4 alcoholic beverages per day and has a distant smoking history. Her temperature is 98.6°F (37°C), blood pressure is 130/75 mmHg, pulse is 90/min, and respirations are 18/min. A firm palpable mass in the upper outer quadrant of the right breast is noted on physical exam. Further workup reveals invasive ductal adenocarcinoma. She eventually undergoes radical resection and is started on a medication that is known to inhibit thymidylate synthetase. This patient is at increased risk for which of the following medication adverse effects?
Q194
A 26-year-old male currently undergoing standard therapy for a recently diagnosed active tuberculosis infection develops sudden onset of fever and oliguria. Laboratory evaluations demonstrate high levels of eosinophils in both the blood and urine. Which of the following is most likely responsible for the patient's symptoms?
Q195
A 65-year-old obese woman presents with changes in her left breast. The patient states that, about a month ago, she noticed that she was able to feel a hard mass in the upper outer quadrant of her left breast, which has not gone away. In addition, her nipple and skin overlying the breast have started to look different. Past medical history is significant for polycystic ovarian syndrome (PCOS) and hypertension, well-managed with lisinopril. The patient has never been pregnant. Menopause was at age 53. Family history is significant for breast cancer in her mother at age 55, and her father who died of lung cancer at age 52. A review of systems is significant for a 13.6 kg (30 lb) weight loss in the last 2 months despite no change in diet or activity. Vitals include: temperature 37.0°C (98.6°F), blood pressure 120/75 mm Hg, pulse 97/min, respiratory rate 16/min, and oxygen saturation 99% on room air. Physical examination is significant for a palpable, hard, fixed mass in the upper outer quadrant of the left breast, as well as nipple retraction and axillary lymphadenopathy. Mammography of the left breast reveals a spiculated mass in the upper outer quadrant. A biopsy confirms invasive ductal carcinoma. Molecular analysis reveals that the tumor cells are positive for HER2/neu receptor, which is associated with a poor prognosis. Which of the following is indicated as first-line targeted therapy for this patient's treatment?
Q196
A 23-year-old man comes to the physician because of a 1-week history of muscle ache, fatigue, and fever that occurs every 2 days. He recently returned from a trip to Myanmar. A peripheral blood smear shows erythrocytes with brick-red granules. The physician recommends a combination of two antimicrobial drugs after confirming normal glucose-6-phosphate dehydrogenase activity. Which of the following is the most appropriate rationale for dual therapy?
Q197
A 25-year-old medical student is doing an international health elective in the Amazon River basin studying tropical disease epidemiology. As part of his pre-trip preparation, he wants to be protected from malaria and is researching options for prophylaxis. Which of the following agents should be avoided for malarial prophylaxis in this patient?
Q198
A 65-year-old man presents with a small painless ulcer with a raised border on his right forearm which has persisted for the last 3 weeks. His past history is significant for 3 occurrences of basal cell carcinoma on different areas of the body during the last 4 years, which have all been surgically excised. The morphology of the present lesion is also highly suggestive of basal cell carcinoma. The patient says that, if the lesion is a basal cell carcinoma, he does not want to undergo biopsy and surgery if it can be avoided. The patient is prescribed a cream, which is FDA-approved for the treatment of small superficial basal cell carcinomas in low-risk areas. The cream contains a chemotherapeutic agent, which is an antimetabolite and an S-phase-specific anticancer drug. Which of the following best explains the mechanism of action of this cream?
Q199
A 55-year-old man presents to his primary care physician for a regular check-up. The patient was born in Germany in 1960 with shortened limbs, underdeveloped digits, absent external ears, and a cleft palate. He is currently in a wheelchair. His past medical history is also notable for hypertension and allergies. He takes lisinopril daily and loratadine as needed. His mother had a complicated past medical history and took multiple medications during her pregnancy. His temperature is 98.6°F (37°C), blood pressure is 120/80 mmHg, pulse is 90/min, and respirations are 20/min. The drug that most likely caused this patient's condition is also indicated for which of the following?
Q200
A 30-year-old man who was recently placed on TMP-SMX for a urinary tract infection presents to urgent care with a new rash. The vital signs include: blood pressure 121/80 mm Hg, pulse 91/min, respiratory rate 18/min, and temperature 36.7°C (98.2°F). Physical examination reveals a desquamative skin covering both of his lower extremities. A basic chemistry panel reveal sodium 139 mmol/L, potassium 3.8 mmol/L, chloride 110 mmol/L, carbon dioxide 47, blood urea nitrogen 23 mg/dL, creatinine 0.9 mg/dL, and glucose 103 mg/dL. Which of the following is the most likely diagnosis?
Antimicrobials US Medical PG Practice Questions and MCQs
Question 191: A cell biologist is studying the activity of a novel chemotherapeutic agent against a cancer cell line. After incubation with the agent and cell detachment from the tissue culture plate, the DNA is harvested from the cells and run on a gel. Of note, there are large bands at every multiple of 180 base pairs on the gel. Which of the following explains the pathophysiology of this finding?
A. Protein denaturation
B. Release of lysosomal enzymes
C. Cellular swelling
D. ATP depletion
E. Caspase activation (Correct Answer)
Explanation: ***Caspase activation***
- The presence of large bands at multiples of **180 base pairs** on a gel indicates a characteristic ladder-like fragmentation pattern of DNA. This fragmentation is a hallmark of **apoptosis**, a form of programmed cell death.
- **Caspase activation**, particularly that of **endonucleases** like caspase-activated DNase (CAD), is responsible for cleaving DNA between nucleosomes, leading to these distinct 180-bp fragments.
*Protein denaturation*
- **Protein denaturation** involves the unfolding of proteins due to stressors but does not directly cause DNA fragmentation into specific band sizes.
- While it can be a part of apoptosis or necrosis, it's not the primary mechanism explaining the observed **DNA laddering**.
*Release of lysosomal enzymes*
- **Lysosomal enzymes** are typically released during **necrosis** or severe cellular injury, leading to widespread, indiscriminate degradation of cellular components, including DNA.
- This degradation would result in a **smear** rather than discrete bands on a gel, as the DNA would be randomly fragmented.
*Cellular swelling*
- **Cellular swelling** is an early, reversible sign of cell injury often associated with **necrosis** or hydropic change due to ion pump dysfunction.
- It does not directly lead to **DNA fragmentation** or the specific laddering pattern seen in apoptosis.
*ATP depletion*
- **ATP depletion** is a critical event in cell injury, often leading to activation of anaerobic glycolysis, failure of ion pumps, and ultimately cell death.
- While **ATP depletion** can contribute to necrosis, apoptosis often requires ATP for the energy-dependent cascade of caspase activation and DNA fragmentation.
Question 192: A 39-year-old man comes to the physician for a follow-up examination. He was diagnosed with latent tuberculosis infection 3 months ago. He has had generalized fatigue and dyspnea on exertion for the past 6 weeks. He does not smoke and drinks 2–3 beers on weekends. Vital signs are within normal limits. Examination shows conjunctival pallor. Laboratory studies show:
Hemoglobin 7.8 g/dL
Mean corpuscular volume 72 μm3
Red cell distribution width 17% (N = 13–15)
Reticulocyte count 0.7%
Leukocyte count 6,800/mm3
Platelet count 175,000/mm3
Serum
Creatinine 0.8 mg/dL
Iron 246 μg/dL
Ferritin 446 ng/mL
Total iron-binding capacity 212 μg/dL (N = 250–450)
Which of the following is the most likely cause of this patient's symptoms?
A. Iron deficiency
B. Vitamin B12 deficiency
C. Beta thalassemia minor
D. Chronic inflammation
E. Adverse effect of medication (Correct Answer)
Explanation: ***Adverse effect of medication***
- The patient was recently diagnosed with **latent tuberculosis** and would likely have been started on **isoniazid** for treatment. Isoniazid is a well-known cause of **sideroblastic anemia** through interference with **pyridoxine (vitamin B6)** metabolism.
- Vitamin B6 is an essential cofactor for **δ-aminolevulinic acid (ALA) synthase**, the rate-limiting enzyme in heme synthesis. Isoniazid-induced B6 deficiency leads to **defective heme synthesis**, resulting in **iron accumulation** in mitochondria of developing erythrocytes (sideroblasts).
- This produces the classic pattern of **microcytic anemia** with **markedly elevated serum iron** (246 μg/dL), **high ferritin** (446 ng/mL), and **low TIBC** (212 μg/dL) due to iron overload. The **low reticulocyte count** (0.7%) indicates inadequate bone marrow response.
*Iron deficiency*
- While this also causes **microcytic anemia** (MCV 72 fL) and **elevated RDW**, the iron studies would show the **opposite pattern**: **low serum iron**, **low ferritin**, and **high TIBC** (>400 μg/dL).
- The patient's **very high iron** (246 μg/dL) and **ferritin** (446 ng/mL) definitively rule out iron deficiency.
*Vitamin B12 deficiency*
- **Vitamin B12 deficiency** causes **macrocytic anemia** (MCV >100 fL), not the **microcytic anemia** seen here (MCV 72 fL).
- B12 deficiency would also typically show **hypersegmented neutrophils** and **elevated MCV**, neither of which fit this presentation.
*Beta thalassemia minor*
- **Beta thalassemia minor** does present with **microcytic anemia** and usually has a **normal or low-normal RDW** (in contrast to the elevated RDW here).
- However, thalassemia trait typically shows **normal or slightly low ferritin** and **normal iron studies**, not the marked **iron overload** pattern seen in this patient.
- Additionally, thalassemia is a **lifelong genetic condition**, not an acute presentation developing over 6 weeks.
*Chronic inflammation*
- **Anemia of chronic disease** typically presents with **normocytic** or **mildly microcytic anemia**, **low serum iron**, **high ferritin**, and **low TIBC** due to hepcidin-mediated iron sequestration.
- The key distinguishing feature is that this patient has **very high serum iron** (246 μg/dL), which is **not consistent** with anemia of chronic disease, where iron is sequestered in macrophages and serum iron remains low.
Question 193: A 65-year-old G4P4 woman presents to her primary care physician complaining of a breast lump. She reports that she felt the lump while conducting a breast self-examination. Her past medical history is notable for endometrial cancer status post radical hysterectomy. She takes aspirin and fish oil. The patient drinks 3-4 alcoholic beverages per day and has a distant smoking history. Her temperature is 98.6°F (37°C), blood pressure is 130/75 mmHg, pulse is 90/min, and respirations are 18/min. A firm palpable mass in the upper outer quadrant of the right breast is noted on physical exam. Further workup reveals invasive ductal adenocarcinoma. She eventually undergoes radical resection and is started on a medication that is known to inhibit thymidylate synthetase. This patient is at increased risk for which of the following medication adverse effects?
A. Mucositis/stomatitis (Correct Answer)
B. Severe diarrhea
C. Myelosuppression
D. DPD deficiency-related severe toxicity
E. Hand-foot syndrome (palmar-plantar erythrodysesthesia)
Explanation: ***Mucositis/stomatitis***
- The patient is likely undergoing treatment with **5-fluorouracil (5-FU)**, a thymidylate synthase inhibitor used for breast cancer.
- **Mucositis** and stomatitis are very common and often dose-limiting side effects of 5-FU, affecting rapidly dividing cells in the oral and gastrointestinal mucosa.
*Severe diarrhea*
- While diarrhea can occur with 5-FU, **severe diarrhea** is more characteristic of **irinotecan** or other topoisomerase I inhibitors.
- The patient's presentation with a breast mass and subsequent treatment with a **thymidylate synthase inhibitor** points away from irinotecan as the primary agent.
*Myelosuppression*
- **Myelosuppression** (bone marrow suppression) is a common side effect of many chemotherapeutic agents, including 5-FU.
- However, **mucositis/stomatitis** is a more prominent and often dose-limiting toxicity for 5-FU compared to myelosuppression, especially when considering the specificity of the question.
*DPD deficiency-related severe toxicity*
- **Dihydropyrimidine dehydrogenase (DPD) deficiency** can lead to severe and potentially fatal toxicity with **5-FU**, but this is related to a genetic predisposition, not a general adverse effect *risk* for all patients receiving the drug.
- The question asks about an increased risk for a general adverse effect, not a specific genetic susceptibility.
*Hand-foot syndrome (palmar-plantar erythrodysesthesia)*
- **Hand-foot syndrome** is a side effect of some chemotherapies, particularly **capecitabine** (an oral prodrug of 5-FU) and **liposomal doxorubicin**.
- While capecitabine is related, the question asks about a medication that inhibits thymidylate synthetase (which 5-FU directly does), and mucositis is a more universal and significant side effect of 5-FU itself.
Question 194: A 26-year-old male currently undergoing standard therapy for a recently diagnosed active tuberculosis infection develops sudden onset of fever and oliguria. Laboratory evaluations demonstrate high levels of eosinophils in both the blood and urine. Which of the following is most likely responsible for the patient's symptoms?
A. Isoniazid
B. Pyrazinamide
C. Rifampin (Correct Answer)
D. Return of active tuberculosis symptoms secondary to patient non-compliance with anti-TB regimen
E. Ethambutol
Explanation: ***Rifampin***
- **Rifampin-induced interstitial nephritis** can present with **fever**, **oliguria**, and **eosinophilia** (in both blood and urine), typically within weeks to months of starting treatment.
- This drug commonly causes hypersensitivity reactions affecting the kidneys, leading to **acute kidney injury**.
*Isoniazid*
- While isoniazid can cause **hepatotoxicity**, **peripheral neuropathy**, and **drug-induced lupus erythematosus**, it is not typically associated with **eosinophilic interstitial nephritis** or **oliguria**.
- Its adverse effects do not usually include the specific constellation of symptoms seen in this patient.
*Pyrazinamide*
- Pyrazinamide is known to cause **hepatotoxicity** (similar to isoniazid) and **hyperuricemia**, which can lead to **gouty arthritis**.
- It does not commonly cause **eosinophilic interstitial nephritis** or conditions presenting with **fever**, **oliguria**, and **eosinophilia**.
*Return of active tuberculosis symptoms secondary to patient non-compliance with anti-TB regimen*
- A return of active tuberculosis would typically manifest as worsening respiratory symptoms, constitutional symptoms (weight loss, night sweats), and possibly new infiltrates on chest X-ray, not primarily acute **fever**, **oliguria**, and **eosinophilia**.
- The presented symptoms are more indicative of a **drug-induced hypersensitivity reaction** rather than a relapse of infection.
*Ethambutol*
- The most significant and well-known adverse effect of ethambutol is **optic neuritis**, leading to **vision impairment** and **color blindness**.
- It is not typically associated with **fever**, **oliguria**, or **eosinophilia** indicative of interstitial nephritis.
Question 195: A 65-year-old obese woman presents with changes in her left breast. The patient states that, about a month ago, she noticed that she was able to feel a hard mass in the upper outer quadrant of her left breast, which has not gone away. In addition, her nipple and skin overlying the breast have started to look different. Past medical history is significant for polycystic ovarian syndrome (PCOS) and hypertension, well-managed with lisinopril. The patient has never been pregnant. Menopause was at age 53. Family history is significant for breast cancer in her mother at age 55, and her father who died of lung cancer at age 52. A review of systems is significant for a 13.6 kg (30 lb) weight loss in the last 2 months despite no change in diet or activity. Vitals include: temperature 37.0°C (98.6°F), blood pressure 120/75 mm Hg, pulse 97/min, respiratory rate 16/min, and oxygen saturation 99% on room air. Physical examination is significant for a palpable, hard, fixed mass in the upper outer quadrant of the left breast, as well as nipple retraction and axillary lymphadenopathy. Mammography of the left breast reveals a spiculated mass in the upper outer quadrant. A biopsy confirms invasive ductal carcinoma. Molecular analysis reveals that the tumor cells are positive for HER2/neu receptor, which is associated with a poor prognosis. Which of the following is indicated as first-line targeted therapy for this patient's treatment?
A. Raloxifene
B. Tamoxifen
C. Goserelin
D. Anastrozole
E. Trastuzumab (Correct Answer)
Explanation: ***Trastuzumab***
- The patient's tumor is **positive for HER2/neu receptor**, which is a direct indication for **trastuzumab (Herceptin)**. This monoclonal antibody specifically targets the HER2 receptor, inhibiting the growth of HER2-driven cancer cells.
- Trastuzumab is considered **first-line targeted therapy** for HER2-positive breast cancer, often used in combination with chemotherapy, and is known to improve survival outcomes in these patients.
*Raloxifene*
- **Raloxifene** is a **selective estrogen receptor modulator (SERM)** primarily used to treat and prevent **osteoporosis** in postmenopausal women, and for the prevention of **estrogen receptor (ER)-positive breast cancer**.
- It would not be indicated as a first-line targeted therapy for this patient because her cancer is **HER2-positive**, not explicitly ER-positive (though ER status is not provided, HER2 positivity is the guiding factor for specific targeted therapy).
*Tamoxifen*
- **Tamoxifen** is also a **selective estrogen receptor modulator (SERM)**, used in the treatment of **estrogen receptor (ER)-positive breast cancer** in both pre- and postmenopausal women.
- It works by blocking estrogen's effects on breast cancer cells, which is not the primary mechanism of action required for a **HER2-positive tumor**.
*Goserelin*
- **Goserelin** is a **gonadotropin-releasing hormone (GnRH) agonist** that suppresses ovarian function, reducing estrogen production. It is used in premenopausal women with **hormone receptor-positive breast cancer**.
- This patient is postmenopausal, and her cancer is **HER2-positive**, not necessarily hormone receptor-positive, making goserelin an inappropriate choice for targeted therapy.
*Anastrozole*
- **Anastrozole** is an **aromatase inhibitor** used in postmenopausal women with **hormone receptor-positive breast cancer** to block the production of estrogen in peripheral tissues.
- While the patient is postmenopausal, her cancer's defining characteristic for targeted therapy is its **HER2-positivity**, not explicitly its hormone receptor status. Aromatase inhibitors are not targeted therapy for HER2-positive disease.
Question 196: A 23-year-old man comes to the physician because of a 1-week history of muscle ache, fatigue, and fever that occurs every 2 days. He recently returned from a trip to Myanmar. A peripheral blood smear shows erythrocytes with brick-red granules. The physician recommends a combination of two antimicrobial drugs after confirming normal glucose-6-phosphate dehydrogenase activity. Which of the following is the most appropriate rationale for dual therapy?
A. Decrease in renal drug secretion
B. Therapy against polymicrobial infections
C. Prevention of infection relapse (Correct Answer)
D. Prevention of drug resistance
E. Decrease in enzymatic drug deactivation
Explanation: ***Prevention of infection relapse***
- The patient's symptoms (fever every 2 days, muscle ache, fatigue after travel to Myanmar) and blood smear findings (**erythrocytes with brick-red granules**, or **Schüffner's dots**, indicating **Plasmodium vivax** or **P. ovale** infection) strongly suggest **malaria**.
- Dual therapy in this context typically involves a **blood-stage antimalaria** (e.g., chloroquine, artemisinin combination therapy) to treat acute symptoms and a **hypnozoiticidal drug** (e.g., **primaquine**) to clear dormant liver stages (**hypnozoites**) of P. vivax/ovale and prevent relapse.
*Decrease in renal drug secretion*
- This is not a primary reason for dual therapy in malaria; drug secretion rates are generally considered when determining individual drug dosages, not for combining multiple drugs.
- While drug interactions can affect renal clearance, it's not the rationale for dual antimalarial use in this common clinical scenario.
*Therapy against polymicrobial infections*
- The presentation is classic for **monomicrobial malaria** due to Plasmodium vivax or ovale, not a polymicrobial bacterial or fungal infection.
- Antimicrobial combinations for polymicrobial infections target different classes of microbes, which is not the case here.
*Prevention of drug resistance*
- While combination therapy is crucial for **preventing drug resistance** in infections like TB or HIV, and also for blood-stage malarial treatment (e.g., artemisinin combination therapy), the specific dual therapy mentioned (blood-stage drug + primaquine) is primarily for **eradicating hypnozoites** to prevent relapse, not solely for preventing blood-stage drug resistance.
- However, in cases of **falciparum malaria**, combination therapy is indeed used to prevent resistance. The brick-red granules, though, point to non-falciparum malaria.
*Decrease in enzymatic drug deactivation*
- This concept relates to pharmacokinetic interactions where one drug might inhibit enzymes that metabolize another, increasing its effectiveness or toxicity.
- This is not the primary purpose of combining a blood-stage antimalarial with primaquine; the primary goal is to target multiple life stages of the parasite.
Question 197: A 25-year-old medical student is doing an international health elective in the Amazon River basin studying tropical disease epidemiology. As part of his pre-trip preparation, he wants to be protected from malaria and is researching options for prophylaxis. Which of the following agents should be avoided for malarial prophylaxis in this patient?
A. Doxycycline
B. Mefloquine
C. Atovaquone-proguanil
D. Quinine
E. Chloroquine (Correct Answer)
Explanation: ***Chloroquine***
- Chloroquine is generally ineffective for malaria prophylaxis in regions with **chloroquine-resistant strains**, which includes most of the **Amazon River basin**.
- Widespread resistance, particularly from *Plasmodium falciparum*, makes it a poor choice for travelers to many endemic areas.
*Doxycycline*
- **Doxycycline** is a highly effective and commonly used agent for malaria prophylaxis in areas with **drug-resistant malaria**.
- It is taken daily and should be started 1-2 days before travel and continued for 4 weeks after leaving the endemic area.
*Mefloquine*
- **Mefloquine** is a good option for prophylaxis in areas with **multidrug-resistant *P. falciparum***, effective even in regions with high chloroquine resistance.
- However, it has significant central nervous system **side effects**, including psychiatric and neurological issues, which may limit its use in some individuals.
*Atovaquone-proguanil*
- **Atovaquone-proguanil** (Malarone) is an effective and well-tolerated prophylactic agent for malaria, including in areas with **chloroquine-resistant strains**.
- It has a convenient dosing schedule (started 1-2 days before travel and continued for only 7 days after leaving) and relatively few side effects.
*Quinine*
- **Quinine** is an effective **treatment** for malaria but is generally not recommended for prophylaxis due to its **short half-life** and significant **side effects** even at prophylactic doses (e.g., cinchonism).
- Its use is typically reserved for the treatment of acute, severe malaria episodes.
Question 198: A 65-year-old man presents with a small painless ulcer with a raised border on his right forearm which has persisted for the last 3 weeks. His past history is significant for 3 occurrences of basal cell carcinoma on different areas of the body during the last 4 years, which have all been surgically excised. The morphology of the present lesion is also highly suggestive of basal cell carcinoma. The patient says that, if the lesion is a basal cell carcinoma, he does not want to undergo biopsy and surgery if it can be avoided. The patient is prescribed a cream, which is FDA-approved for the treatment of small superficial basal cell carcinomas in low-risk areas. The cream contains a chemotherapeutic agent, which is an antimetabolite and an S-phase-specific anticancer drug. Which of the following best explains the mechanism of action of this cream?
A. Inhibition of DNA repair
B. Inhibition of de novo purine nucleotide synthesis
C. Inhibition of dihydrofolate reductase
D. Inhibition of ribonucleotide reductase
E. Inhibition of thymidylate synthase (Correct Answer)
Explanation: ***Inhibition of thymidylate synthase***
- The cream contains **5-fluorouracil (5-FU)**, which is an **antimetabolite** and an **S-phase-specific** chemotherapeutic agent.
- 5-FU is converted to 5-fluorodeoxyuridine monophosphate (5-FdUMP), which **irreversibly inhibits thymidylate synthase**, thereby blocking the synthesis of deoxythymidine triphosphate (dTTP) and ultimately **DNA synthesis**.
*Inhibition of DNA repair*
- While some chemotherapeutic agents affect DNA repair, this is not the primary mechanism of action for **antimetabolites targeting S-phase DNA synthesis**.
- Drugs like alkylating agents or platinum compounds primarily damage DNA, and their effects might indirectly involve DNA repair pathways.
*Inhibition of de novo purine nucleotide synthesis*
- This is the mechanism of action for drugs like **6-mercaptopurine** or **azathioprine**, which are purine analogs.
- The patient is described as receiving an antimetabolite that affects DNA synthesis in the S-phase, which points more directly to pyrimidine synthesis inhibition for 5-FU.
*Inhibition of dihydrofolate reductase*
- This is the mechanism of action for **methotrexate**, which is an antimetabolite that inhibits the conversion of dihydrofolate to tetrahydrofolate.
- This inhibition prevents the synthesis of purines and thymidylate, but it is not the direct mechanism of action for the drug used in this context (5-FU).
*Inhibition of ribonucleotide reductase*
- This is the mechanism of action for drugs like **hydroxyurea** or **gemcitabine**.
- Ribonucleotide reductase converts ribonucleotides to deoxyribonucleotides, which are essential for DNA synthesis, but this is a separate target from thymidylate synthase.
Question 199: A 55-year-old man presents to his primary care physician for a regular check-up. The patient was born in Germany in 1960 with shortened limbs, underdeveloped digits, absent external ears, and a cleft palate. He is currently in a wheelchair. His past medical history is also notable for hypertension and allergies. He takes lisinopril daily and loratadine as needed. His mother had a complicated past medical history and took multiple medications during her pregnancy. His temperature is 98.6°F (37°C), blood pressure is 120/80 mmHg, pulse is 90/min, and respirations are 20/min. The drug that most likely caused this patient's condition is also indicated for which of the following?
A. Acne vulgaris
B. Multiple myeloma (Correct Answer)
C. Deep venous thrombosis
D. Recurrent miscarriage
E. Bipolar disease
Explanation: ***Multiple myeloma***
- The patient's presentation with **shortened limbs**, **underdeveloped digits**, **absent external ears**, and a **cleft palate**, born in Germany in 1960, is highly characteristic of **thalidomide embryopathy**.
- **Thalidomide**, the drug responsible for these birth defects, is currently used in adults for treating **multiple myeloma** due to its anti-angiogenic and immunomodulatory properties.
*Acne vulgaris*
- **Thalidomide** is not indicated for the treatment of **acne vulgaris**.
- **Acne vulgaris** is typically treated with retinoids (topical or oral), antibiotics, or hormonal therapies.
*Deep venous thrombosis*
- While certain medications can cause or prevent **deep venous thrombosis (DVT)**, **thalidomide** itself is not primarily indicated for DVT treatment or prevention.
- In fact, Thalidomide has been associated with an **increased risk of thrombosis**, especially when combined with steroids, which often necessitates DVT prophylaxis when used for multiple myeloma.
*Recurrent miscarriage*
- **Thalidomide** is a severe **teratogen** and would absolutely not be used to treat or prevent recurrent miscarriage.
- Its use during pregnancy can lead to severe developmental anomalies, as seen in the patient's history.
*Bipolar disease*
- **Thalidomide** has no established role in the treatment of **bipolar disease**.
- Mood stabilizers such as lithium, valproate, lamotrigine, and atypical antipsychotics are common treatments for bipolar disorder.
Question 200: A 30-year-old man who was recently placed on TMP-SMX for a urinary tract infection presents to urgent care with a new rash. The vital signs include: blood pressure 121/80 mm Hg, pulse 91/min, respiratory rate 18/min, and temperature 36.7°C (98.2°F). Physical examination reveals a desquamative skin covering both of his lower extremities. A basic chemistry panel reveal sodium 139 mmol/L, potassium 3.8 mmol/L, chloride 110 mmol/L, carbon dioxide 47, blood urea nitrogen 23 mg/dL, creatinine 0.9 mg/dL, and glucose 103 mg/dL. Which of the following is the most likely diagnosis?
A. Atopic dermatitis
B. Steven-Johnson syndrome (SJS) (Correct Answer)
C. Dermatitis herpetiformis
D. Seborrheic dermatitis
E. Toxic epidermal necrolysis (TEN)
Explanation: ***Steven-Johnson syndrome (SJS)***
- The presentation of a **desquamative rash** following recent administration of **TMP-SMX (trimethoprim-sulfamethoxazole)**, a known culprit drug, is highly suggestive of SJS.
- SJS is a severe form of **cutaneous adverse drug reaction** involving mucocutaneous lesions and epidermal detachment, typically less than 10% of the body surface area.
*Atopic dermatitis*
- Characterized by **eczematous, pruritic lesions** that are often chronic and recurrent.
- It is not typically associated with acute, widespread **desquamation** following drug exposure.
*Dermatitis herpetiformis*
- Presents with **pruritic, vesicular lesions** primarily on extensor surfaces.
- It is strongly associated with **celiac disease** and is not typically drug-induced.
*Seborrheic dermatitis*
- Manifests as **erythematous patches** with greasy scales, commonly affecting areas rich in sebaceous glands مثل the scalp, face, and chest.
- It does not involve acute **desquamation** or a clear association with TMP-SMX.
*Toxic epidermal necrolysis (TEN)*
- While TEN is also a severe cutaneous adverse drug reaction, it involves **extensive epidermal detachment** covering more than 30% of the body surface area.
- This patient's rash, described as covering both lower extremities, is more consistent with SJS due to the lesser extent of involvement.
