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Answer and proper diagnosis
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Here is the complete, corrected answer to all 6 questions in the exercise:
Exercise MI 7.2.2 - Case 1: Full Model Answer
Case Summary
A 35-year-old woman with multiple sexual partners presenting with painful vesicular/ulcerative lesions on the genitalia (vulva & cervix) + itching + enlarged, tender, firm inguinal lymph nodes.
Q1. Most Probable Diagnosis
Primary Syphilis - caused by Treponema pallidum
Note: The answer written in the image ("Syphilis") is correct. However, the classic primary syphilis chancre is usually painless (firm, indurated, painless ulcer). Since this patient has painful ulcers, Genital Herpes (HSV-2) is actually the more common cause of painful genital ulcers. The combination of VDRL/RPR testing shown in the images points the exercise toward Syphilis. In a real clinical scenario, both must be considered and tested for.
Most likely diagnosis based on serology shown: Syphilis (Primary)
- Causative organism: Treponema pallidum
- The VDRL and RPR-Carbon tests shown are both reactive, confirming syphilis serology.
Q2. Sample Collection and Laboratory Techniques
| Method | Details |
|---|---|
| Dark-field microscopy | Scrape the ulcer base/chancre; collect serous exudate on a slide; examine immediately for motile spirochetes (T. pallidum) - the gold standard for primary syphilis |
| PCR (NAAT) | Swab from ulcer base; detects T. pallidum DNA - highly sensitive |
| Pus/exudate collection | From genital ulcer or lymph node aspirate (bubo) |
| Serum (blood) | For VDRL, RPR, FTA-ABS, TPPA serological tests |
| Tissue biopsy | For silver staining (Warthin-Starry stain) to visualize spirochetes |
The student's answer ("pus collection, swab/blister") is partially correct but incomplete - dark-field microscopy is the key technique that should have been mentioned.
Q3. Identify and Interpret the Tests (Serum)
The two tests shown in the images are:
VDRL (Venereal Disease Research Laboratory Test)
- A lipoidal/non-treponemal test
- Detects IgG and IgM antibodies against cardiolipin-lecithin-cholesterol antigen complex (NOT direct antibodies to T. pallidum)
- Used for: screening and monitoring treatment response (titres fall with treatment)
- The VDRL shown at dilutions 1:8, 1:16, 1:32 indicates quantitative/titred testing
RPR-Carbon (Rapid Plasma Reagin)
- Also a non-treponemal/lipoidal test
- Easier to perform; uses unheated serum - preferred for rapid screening
- Carbon particles make agglutination visible macroscopically
Interpretation:
Both tests detect reagin antibodies (non-specific lipoidal antibodies) - they do NOT directly detect T. pallidum. A reactive result must be confirmed with a treponemal test (FTA-ABS or TPPA) to rule out biological false positives.
The student's answer ("VDRL/RPR detect reagin antibodies") is correct.
- Harrison's Principles of Internal Medicine 22E, p. 1476
Q4. Causes of Genital Ulcers and Organisms
| Disease | Causative Organism | Ulcer Character |
|---|---|---|
| Syphilis | Treponema pallidum | Painless, indurated, single chancre |
| Genital Herpes | HSV-2 (or HSV-1) | Painful, multiple vesicles/ulcers |
| Chancroid | Haemophilus ducreyi | Painful, soft, ragged edge, suppurative lymphadenopathy |
| Lymphogranuloma Venereum (LGV) | Chlamydia trachomatis (L1-L3) | Small painless papule/ulcer + large painful buboes |
| Donovanosis (Granuloma Inguinale) | Klebsiella granulomatis | Painless, beefy-red, friable lesion |
The student's answer is partially correct (listed herpes + syphilis only) - the full list above should be known.
Q5. Biological False Positive (BFP) in VDRL/RPR
A biological false positive = a reactive VDRL/RPR in a person who does NOT have syphilis (and treponemal tests are negative).
This occurs because the lipid antigens in these tests cross-react with human tissue antigens in certain conditions.
Causes of BFP:
Acute BFP (titre usually low, < 1:8, resolves in <6 months):
- Malaria (the example given by the student - correct)
- Viral infections: EBV (infectious mononucleosis), HIV, hepatitis, varicella
- Vaccinations
- Pregnancy
Chronic BFP (titre persists >6 months):
- Tuberculosis (TB) - the student correctly listed this
- Autoimmune diseases: SLE (most common), rheumatoid arthritis, antiphospholipid syndrome
- Leprosy
- Intravenous drug use
- Thyroiditis, liver disease
The student's answer (+ve VDRL without syphilis, e.g., malaria, TB) is correct, though the categories of acute vs. chronic BFP should be distinguished.
- Harrison's Principles of Internal Medicine 22E - "False-Positive Serologic Tests for Syphilis"
Q6. Significant Titre of VDRL/RPR
A titre of ≥ 1:8 is considered significant and suggestive of active syphilis.
- Titre ≥ 1:32 is commonly seen in secondary syphilis
- A fourfold rise (e.g., 1:8 to 1:32) indicates active/new infection or reinfection
- A fourfold fall (e.g., 1:32 to 1:8) after treatment = adequate treatment response
- Titres ≤ 1:8 may indicate latent/treated disease OR biological false positive
The student's answer ("≥ 1:8 = significant") is correct.
- Harrison's Principles of Internal Medicine 22E, p. 1476
Summary of Student's Answers
| Question | Student Answer | Assessment |
|---|---|---|
| Q1 - Diagnosis | Syphilis | ✅ Correct (note: HSV is more classically painful) |
| Q2 - Sample collection | Pus collection, swab | ⚠️ Partial - missing dark-field microscopy |
| Q3 - Tests | VDRL/RPR detect reagin antibodies | ✅ Correct |
| Q4 - Causes of genital ulcer | Genital herpes (HSV-2), Syphilis → T. pallidum | ⚠️ Partial - missing chancroid, LGV, donovanosis |
| Q5 - Biological false positive | +ve VDRL without syphilis, e.g., malaria, TB | ✅ Correct |
| Q6 - Significant titre | ≥ 1:8 = significant | ✅ Correct |
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