What is the cause of pinpoint papules (papular eruptions) on a 5-year-old child, initially presenting on the thighs and spreading in a cephalocaudal (head-to-tail) direction without any additional symptoms?

Differential Diagnosis for Pin Point Papules in a 5-Year-Old

Single Most Likely Diagnosis

• Molluscum contagiosum: This is a common, highly contagious viral skin infection that presents with small, pinpoint to pea-sized papules. It often starts in one area and spreads to other parts of the body, including a cephalocaudal distribution. The lack of other symptoms such as fever, itching, or systemic illness makes this diagnosis more likely.

Other Likely Diagnoses

• Keratosis pilaris: A benign condition characterized by small, rough, sandpapery papules often on the thighs, arms, and buttocks. It can spread but typically does not have a distinct pattern like cephalocaudal.
• Folliculitis: An inflammation of the hair follicles, which can present with small papules or pustules. It can occur anywhere on the body with hair follicles and might spread, but it often is accompanied by signs of inflammation or infection.
• Pityriasis rosea: Although it typically starts with a herald patch followed by a wider rash, the secondary lesions can be small and pinpoint, spreading in a Christmas tree distribution, which might be confused with a cephalocaudal spread.

Do Not Miss Diagnoses

• Scabies: Caused by the mite Sarcoptes scabiei, it can present with small papules and intense itching, often sparing the face in infants but can involve any area in older children. The distribution can be widespread, and the diagnosis is critical due to its contagious nature and the potential for secondary infections.
• Chickenpox (Varicella): Although typically associated with fever and a more widespread rash that progresses from macules to papules, vesicles, pustules, and crusts, the initial stages can be confused with pinpoint papules. The cephalocaudal spread is consistent with varicella.

Rare Diagnoses

• Lichen nitidus: A rare condition characterized by tiny, shiny, flat-topped papules that can appear anywhere on the body. It's more common in children and young adults but is less likely due to its rarity and the fact that it doesn't typically spread in a distinct pattern.
• Papular urticaria: A condition caused by an allergic reaction to insect bites, characterized by small papules. While it can occur in children and might spread, the history of insect bites or exposure is usually a key factor in the diagnosis.