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This thing and fever, possible diagnosis

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Before I narrow this down, I need a bit more context since the photo alone doesn't tell me whether this mark is new or an old scar, or whether it's tender/inflamed.
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How long has this small mark/bump on the baby's arm been there? · Is the spot itself red, warm, swollen, tender, or oozing/pus-filled? · Did the baby recently get a BCG or other injection in that arm?

Before I narrow this down, I need a bit more context since the photo alone doesn't tell me whether this mark is new or an old scar, or whether it's tender/inflamed.

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BCG vaccination scar on infant arm

Based on the photo, that small round mark on the baby's upper arm looks like it's sitting right over the deltoid/upper-arm region - the classic site for a BCG vaccination scar (or possibly a healed injection site). It looks fairly calm in the photo (skin-colored, slightly depressed/dimpled center, no obvious redness, pus, or surrounding swelling). Since I don't have your reply on timing/tenderness, here's how to think through it:
Most likely explanations for "this bump" + fever together:
  1. Old, healed BCG scar (incidental) + unrelated fever
    • If this mark has been there for weeks/months and looks exactly like the photo (flat, painless, no redness), it's almost certainly just the normal healed BCG vaccination scar. BCG scars form 2-3 weeks after the shot and heal within about 3 months, leaving a small pale, sometimes slightly pitted mark - Harrison's Principles of Internal Medicine 22E.
    • In that case, the fever is very likely from something else entirely (common viral infection, teething, ear infection, etc.) and just coincidental with the photo.
  2. Local BCG reaction causing the fever (if the mark is recent, tender, red, swollen, or draining)
    • Ulceration and regional lymphadenitis occur in about 1-10% of BCG vaccinations, and suppurative lymphadenitis, abscess formation, or rarely osteomyelitis can occur, sometimes with fever - Park's Textbook of Preventive and Social Medicine.
    • If there's a lump in the armpit on the same side, or pus/discharge from the site, this needs a doctor's review.
  3. Furuncle/skin abscess - if it's a new, tender, red, warm bump (not at a known vaccination site), a boil from a Staph infection can cause fever, especially in infants.
  4. Molluscum contagiosum - a pearly, dome-shaped papule with a central dimple (umbilication) is classic for this viral skin infection, but it's usually painless and doesn't itself cause fever unless secondarily infected.
What would make this urgent (see a doctor same day):
  • The spot is red, warm, swollen, tender, or draining pus
  • There's a swollen lump in the armpit (axillary node) on the same side
  • Fever is high (>38.5°C), the baby is lethargic, feeding poorly, or fever lasts more than 2-3 days
  • Any spreading redness around the mark
What's reassuring:
  • The mark looks flat/quiet exactly as in your photo, has been there a while, and matches a known vaccination site
  • Baby is otherwise active, feeding well, and fever is low-grade
Since I can't examine tenderness, warmth, or exact duration from a photo, if the fever persists beyond a day or two, or the spot changes (redness, swelling, discharge), please get the baby seen by a pediatrician - a brief exam and, if needed, a CBC can quickly rule in/out a local skin infection versus BCG reaction versus an unrelated viral fever.

This small swelling in one side leg and fever since yesterday on 5 months old

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5 month old baby unilateral leg swelling with fever causes

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"septic arthritis" AND infant AND diagnosis

A 5-month-old with new unilateral leg swelling and fever since yesterday needs a same-day in-person medical evaluation. This combination is a recognized red flag in infants and should not be managed by messaging alone. Here's why and what it could be:
Why this combination is concerning: In infants, fever plus swelling of a limb (especially if the baby isn't moving that leg normally, cries when it's touched, or won't bear weight/kick as usual) is the classic presentation of septic arthritis or osteomyelitis - infection of the joint or bone, most often from Staphylococcus aureus. In neonates and young infants this can present as "pseudoparalysis" - the baby simply stops moving the affected limb, which can look like it's not painful but is actually protecting the joint - Rosen's Emergency Medicine; Bailey and Love's Short Practice of Surgery. This is time-sensitive: delayed treatment can damage the growth plate and joint, so it needs an exam, bloodwork (CBC, CRP/ESR), and often an ultrasound or X-ray of the leg/hip today, not "wait and see."
Other possibilities to consider, roughly in order of likelihood:
  1. Vaccine injection-site reaction - At 5 months, babies typically get a Pentavalent/DPT, PCV, and/or OPV dose, injected into the anterolateral thigh. Local swelling, redness, and fever are recognized side effects, and rarely, swelling of the entire thigh can occur after DTaP-containing vaccines - Red Book 2021; Park's Textbook of Preventive and Social Medicine. If your baby had a vaccination in that leg in the last 1-3 days, this is a strong possibility, especially if the swelling is right at the injection site.
  2. Cellulitis or abscess - a bacterial skin/soft-tissue infection, sometimes from a minor scratch, insect bite, or infected hair follicle, causing local redness, warmth, and fever.
  3. Trauma/fracture - even minor unwitnessed trauma (e.g., leg caught in crib bars, being pulled) can cause swelling; toddler's fractures are less common at 5 months but possible.
  4. Deep vein thrombosis or lymphatic/vascular causes - rare at this age, usually only if there's a central line history, but included for completeness.
Ask yourself before you get to the doctor (this helps triage):
  • Did the baby get an injection in that leg in the last few days? Where exactly is the swelling relative to it?
  • Is the leg red, warm, or shiny-looking, or does the baby scream when you move/touch it?
  • Is the baby still kicking both legs equally, or favoring one side?
  • How high is the fever, and is the baby feeding/alert normally otherwise?
Bottom line: Because septic arthritis/osteomyelitis is a true emergency in a baby this young and can look deceptively mild early on, please have your pediatrician or an ER see the baby today, even if it turns out to be "just" a vaccine reaction. Do not start any antibiotics without a doctor's assessment first, and avoid firmly pressing or manipulating the leg in the meantime.
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