Regarding microbiological culture media, which of the following statements is/are correct? 1. Lowenstein-Jensen medium is an egg-based selective medium used for culture of Mycobacterium tuberculosis. 2. Sabouraud's Dextrose Agar has a pH of 7.4, which favors fungal growth. 3. Thayer-Martin medium contains vancomycin, colistin, and nystatin, making it selective for Neisseria gonorrhoeae. 4. Vibrio cholerae produces blue-green colonies on TCBS agar. A) 1 and 3 only B) 2 and 4 only C) 1, 2 and 3 only D) All of the above
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All four statements are now fully verified from authoritative textbooks. Here is the complete breakdown:
Answer: A) 1 and 3 only
Statement-by-Statement Analysis
Statement 1 - CORRECT ✓
Lowenstein-Jensen (LJ) medium is an egg-based selective medium for Mycobacterium tuberculosis.
Confirmed by Medical Microbiology 9e:
"This medium, used for the isolation of mycobacteria, contains glycerol, potato flour, salts, and coagulated whole eggs (to solidify the medium). Malachite green is added to inhibit gram-positive bacteria."
And from Quick Compendium of Clinical Pathology 5th edition: LJ is listed under "Egg based" media, with the option to add antibiotics to make it selective (Gruft or Mycobactose modifications). Statement 1 is accurate - it is egg-based, and selective (or made selective with additives) for mycobacteria including M. tuberculosis.
Statement 2 - INCORRECT ✗
Sabouraud's Dextrose Agar (SDA) does NOT have a pH of 7.4.
Confirmed by Henry's Clinical Diagnosis and Management by Laboratory Methods:
"The medium used traditionally is Sabouraud dextrose agar, which has a pH of 5.5 to 5.6 and was designed for the isolation of dermatophytic fungi."
The acidic pH of ~5.5 is precisely what favors fungal growth and suppresses bacterial contamination. The Emmons modification raises it to pH 6.8-7.0, but never to 7.4. A pH of 7.4 is neutral/physiological and characteristic of bacteriological media, not fungal media.
Statement 3 - CORRECT ✓
Thayer-Martin medium contains vancomycin, colistin, and nystatin (plus trimethoprim).
Confirmed by Jawetz, Melnick & Adelberg's Medical Microbiology 28e:
"To avoid overgrowth by contaminants, selective media contain antimicrobial drugs (eg, vancomycin, colistin, nystatin, and trimethoprim)."
- Vancomycin - inhibits gram-positive organisms
- Colistin - inhibits gram-negative organisms (except Neisseria)
- Nystatin - inhibits fungi
- Trimethoprim - inhibits Proteus spp. (present in modified Thayer-Martin [MTM])
This makes the medium selective for Neisseria gonorrhoeae (and N. meningitidis). Statement 3 is correct (trimethoprim is in the MTM modification but vancomycin, colistin, and nystatin are the classic three).
Statement 4 - INCORRECT ✗
Vibrio cholerae does NOT produce blue-green colonies on TCBS - it produces YELLOW colonies.
Confirmed by Jawetz, Melnick & Adelberg's Medical Microbiology 28e:
"V. cholerae produces yellow colonies (sucrose fermented) on TCBS agar that are readily visible against the dark-green background of the agar."
And from Quick Compendium of Clinical Pathology 5th edition:
"V. cholerae colonies are yellow because they ferment sucrose."
It is V. parahaemolyticus and V. vulnificus that produce green (blue-green) colonies on TCBS because they do NOT ferment sucrose.
Summary Table
| Statement | Content | Verdict |
|---|---|---|
| 1 | LJ medium - egg-based, for M. tuberculosis | ✅ Correct |
| 2 | SDA - pH 7.4 | ❌ Wrong (pH is 5.5-5.6) |
| 3 | Thayer-Martin - vancomycin, colistin, nystatin | ✅ Correct |
| 4 | V. cholerae - blue-green colonies on TCBS | ❌ Wrong (produces YELLOW colonies) |
Correct answer: A) 1 and 3 only
A 42-year-old man presents with excruciating unilateral periorbital headaches associated with ipsilateral lacrimation and nasal congestion. Each episode lasts about 60 minutes and has occurred twice daily for the past 6 weeks. He reports complete remission for nearly a year before the current episode. High-flow oxygen rapidly aborts his attacks. Which of the following is the most appropriate medication to reduce the frequency of future attacks during the current cluster period? A) Verapamil B) Sodium valproate C) Propranolol D) Flunarizine
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Answer: A) Verapamil
Diagnosis First
This case is classic episodic cluster headache:
- Unilateral severe periorbital pain lasting 45-90 minutes
- Ipsilateral autonomic features (lacrimation, nasal congestion)
- High attack frequency (1-8/day) in a defined "cluster period" (6 weeks here)
- Complete remission between periods (nearly 1 year)
- High-flow oxygen (100% O2 at 12-15 L/min via non-rebreather) is a first-line abortive treatment - its effectiveness here clinches the diagnosis
The question asks specifically about prophylactic/preventive medication to reduce frequency during the current cluster period.
Why Verapamil (A) is correct
From Harrison's Principles of Internal Medicine 22E (2025):
"Most experts favor verapamil as the first-line preventive treatment for patients with chronic cluster headache or with prolonged bouts... Verapamil 160-960 mg/d [for both short-term and long-term prevention of episodic cluster headache]."
From Bradley and Daroff's Neurology in Clinical Practice:
"The calcium channel blockers, particularly verapamil, are considered first-line preventive therapy for both episodic and chronic cluster headache... The initial starting dose of verapamil is 80 mg three times a day... The daily dose can be increased in 40- to 80-mg increments every 7-14 days until the attacks disappear."
Key practical points about verapamil in cluster headache:
- Doses required (up to 720-960 mg/day) are much higher than those used for cardiac conditions
- A baseline ECG is mandatory before starting, and repeated with dose increases, because verapamil can cause AV block by slowing conduction at the AV node
- ~20% of patients develop ECG abnormalities; PR interval must be monitored
- Takes ~2 weeks to become fully effective, so short-term bridging with prednisone is often co-prescribed at initiation
Why the other options are wrong
| Option | Drug | Reason it's wrong |
|---|---|---|
| B | Sodium valproate | Used for migraine prophylaxis; not first-line for cluster headache. May be used as adjunct in refractory chronic cluster, but not the standard answer |
| C | Propranolol | Beta-blocker; first-line preventive for migraine, NOT cluster headache. Has no established role in cluster prophylaxis |
| D | Flunarizine | A calcium channel blocker used for migraine prophylaxis (especially in Europe/Asia); not the drug of choice for cluster headache |
Cluster Headache Preventive Hierarchy (from Harrison's 2025)
| Tier | Drug | Notes |
|---|---|---|
| First-line | Verapamil 160-960 mg/d | For both episodic and chronic |
| Short-term bridge | Prednisone 60 mg tapering over 21 days | While verapamil takes effect |
| Second-line | Lithium 400-800 mg/d | Especially for chronic cluster |
| Adjuncts | Topiramate, melatonin, occipital nerve block | Refractory cases |
| Biologic | Galcanezumab (anti-CGRP mAb) | FDA-approved for episodic cluster |
Verapamil is unambiguously the correct answer - Bradley and Daroff's calls it "first-line preventive therapy" and Harrison's places it at the top of the table for both short-term and long-term prevention of episodic cluster headache.
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