DR5.1,DR6.1 | Ectoparasitic Infestations — Practice Quiz

Correct. Sarcoptes scabiei var. hominis causes scabies. The fertilised female burrows into the stratum corneum, creating the characteristic burrow. Family clustering and nocturnal itch are key epidemiological clues.

Scabies is caused by Sarcoptes scabiei var. hominis, an eight-legged mite (arachnid). The burrow in web spaces is pathognomonic.

The correct answer is B. The burrow in web spaces, nocturnal itch, and family clustering point strongly to Sarcoptes scabiei var. hominis. Pediculus capitis causes head lice; Phthirus pubis causes pubic lice.

Correct. Permethrin 5% cream is the first-line topical scabicide. It must be applied from the neck downwards to the entire body surface (including under nails, between toes, and to genitalia), left for 8-12 hours, then washed off. A second application after 1 week is often recommended.

First-line topical treatment for scabies is permethrin 5% cream applied neck-down for 8-12 hours. Permethrin 1% is used for pediculosis capitis, not scabies.

Correct answer is B. Permethrin 5% cream applied neck-down for 8-12 hours is first-line. Permethrin 1% is for head lice. Lindane is neurotoxic and not recommended in pregnancy or children. Benzyl benzoate is topical, not oral.

Correct. Simultaneous treatment of ALL close contacts (household members, sexual partners) — including those without symptoms — is mandatory. The incubation period is 4-6 weeks in first infestation; asymptomatic contacts are not yet sensitised but carry mites and will reinfest treated individuals.

Treatment failure in scabies most commonly results from not treating all close contacts simultaneously. Even asymptomatic contacts may harbour mites in the pre-sensitisation phase.

Correct answer is B. Treating only symptomatic contacts is the most common reason for scabies treatment failure. Asymptomatic contacts in the pre-sensitisation phase harbour live mites and reinfest treated individuals.

Correct. Crusted (Norwegian) scabies represents hyperinfestation occurring when the immune response is unable to control mite proliferation (as in HIV/AIDS). The thick hyperkeratotic crusts contain thousands to millions of mites, making it extraordinarily contagious. Standard permethrin alone may be insufficient; oral ivermectin is typically combined.

Crusted (Norwegian) scabies is a hyperinfestation in immunocompromised patients (HIV, HTLV-1, organ transplant recipients, elderly/institutionalised). Millions of mites are present; highly contagious to contacts.

Correct answer is C. The combination of immunocompromise (HIV), widespread hyperkeratotic crusted lesions, and highly contagious spread to carers is the classic presentation of crusted (Norwegian) scabies.

Correct. Post-scabetic itch is a type-IV (delayed-type) hypersensitivity reaction to residual mite antigens (dead mites, eggs, faecal pellets) and can persist for 2-4 weeks after successful treatment. The absence of new burrows and absence of active infestation in contacts are the key reassurance clues. Management is oral antihistamines and/or topical corticosteroids, not re-treatment.

Post-scabetic itch from residual type-IV hypersensitivity to dead mite antigens and faecal pellets can persist 2-4 weeks after successful treatment. Distinguishing this from reinfestation prevents unnecessary re-treatment.

Correct answer is C. With no new burrows and no active contacts, persistent itch 2 weeks after treatment is post-scabetic hypersensitivity, not treatment failure. Unnecessary re-treatment adds toxicity without benefit.

Correct. The gold standard for diagnosing active pediculosis capitis is demonstrating live lice. Wet-combing damp, conditioned hair with a fine-toothed (≤0.3 mm) louse comb over white paper is more reliable than visual inspection alone. Dead nits can persist on hair shafts for weeks after successful treatment and should not trigger re-treatment.

Diagnosis of pediculosis capitis requires demonstration of LIVE lice — best achieved by wet-combing damp conditioned hair. Nits alone are insufficient (dead nits persist after successful treatment).

Correct answer is B. Non-viable (dead/empty) nits alone do not confirm active infestation. Live lice must be demonstrated — wet-combing is the most reliable method.

Correct. Pediculosis corporis is unique in that the lice live and lay eggs in clothing seams, not on skin — this is why examining the clothing is mandatory. The maculae ceruleae (bluish-grey macules) result from louse saliva. Crucially, P. corporis is the vector for epidemic typhus (Rickettsia prowazekii), epidemic relapsing fever (Borrelia recurrentis), and trench fever (Bartonella quintana) — making it the most medically significant louse.

Pediculosis corporis (body louse, Pediculus humanus corporis) lives in clothing seams (not on skin), causes maculae ceruleae, and is the vector for epidemic typhus (Rickettsia prowazekii), epidemic relapsing fever (Borrelia recurrentis), and trench fever (Bartonella quintana).

Correct answer is B. The clinical clue is lice and nits in clothing seams (not on hair). P. corporis is the louse with systemic disease-vectoring capability — epidemic typhus, relapsing fever, trench fever.

Correct. Permethrin 1% is first-line for pediculosis capitis. A single application kills live lice but not all eggs (nits); repeating after 7-10 days ensures any newly hatched nymphs are killed before they can mature and lay eggs. The second application is essential to prevent re-establishment of infestation.

First-line treatment for pediculosis capitis is permethrin 1% lotion applied to damp hair, left 10 minutes, rinsed, repeated after 7-10 days (to kill newly hatched nymphs from eggs missed in the first application).

Correct answer is B. A single application of permethrin 1% is not sufficient — eggs (nits) may survive the first treatment. Repeating after 7-10 days is mandatory. Oral ivermectin may be used for resistant/extensive cases but is not first-line.

Correct. Permethrin 5% cream is classified as FDA Category B in pregnancy and is the safest topical scabicide option. It has minimal systemic absorption. Lindane is absolutely contraindicated in pregnancy due to neurotoxicity. Oral ivermectin is not recommended in pregnancy (FDA Category C; potential teratogenicity not fully excluded). Benzyl benzoate at 25% can be irritant; dilution to 12.5% is used in pregnancy as a second-line option.

In pregnancy, permethrin 5% cream is the preferred scabicide. Lindane is contraindicated (neurotoxicity risk). Oral ivermectin is not recommended in pregnancy. Benzyl benzoate is used (diluted to 12.5%) but is second-line.

Correct answer is C. Permethrin 5% cream is the safest option in pregnancy. Lindane is contraindicated. Oral ivermectin should be avoided in pregnancy.

Correct. Pediculosis pubis (crab louse, Phthirus pubis) is predominantly sexually transmitted. Sexual contacts must be identified and treated simultaneously with the index case to prevent re-infestation. The presence of pubic lice in an adolescent warrants consideration of other STDs. In children below the age of sexual activity, pubic lice should raise the possibility of sexual abuse.

Pediculosis pubis in adults/adolescents is sexually transmitted. Sexual contacts must be traced and treated simultaneously. Co-infection with other STDs should be screened. In children, pediculosis pubis may indicate sexual abuse.

Correct answer is B. Pediculosis pubis is an STD in adolescents and adults. Sexual contact tracing, simultaneous treatment, and STD screening are mandatory alongside topical treatment.